diff --git a/Makefile b/Makefile
index 4479078..8a8b030 100644
--- a/Makefile
+++ b/Makefile
@@ -22,16 +22,19 @@
 # Makefile for logsdu - build the extension for Firefox and Chrome.
 #
 # Usage:
-#   make            # typecheck and bundle for Firefox into build/
-#   make chrome     # same, but with Chrome's manifest, into build/
+#   make            # everything: build and package both browsers
+#   make firefox    # build Firefox only -> build/firefox/
+#   make chrome     # build Chrome only  -> build/chrome/
 #   make test       # run the unit tests
 #   make smoke      # check the background bundle works as a service worker
-#   make xpi        # package the Firefox build  -> dist/logsdu-<version>.xpi
-#   make crx        # package the Chrome build   -> dist/logsdu-<version>-chrome.zip
-#   make packages   # both of the above
-#   make clean      # remove build/ and dist/
+#   make xpi        # package Firefox -> dist/logsdu-<version>-firefox.xpi
+#   make crx        # package Chrome  -> dist/logsdu-<version>-chrome.zip
+#   make packages   # both of the above (same as plain "make")
+#   make clean      # remove build/, dist/ and stray packages
+#   make distclean  # clean, plus node_modules/
 #
-# build/ holds the unpacked extension and is regenerated from scratch; dist/
+# build/<target>/ holds the unpacked extension for one browser; the two targets
+# never share a directory, so neither can be left stale by the other. dist/
 # holds the packages meant to be published, and is kept out of build/ so that
 # packaging never tries to include its own output.
 #
@@ -40,22 +43,33 @@
 #
 # Load the unpacked build while developing:
 #   Firefox  about:debugging -> This Firefox -> Load Temporary Add-on ->
-#            build/manifest.json          (dropped when Firefox restarts)
-#   Chrome   chrome://extensions -> Developer mode -> Load unpacked -> build/
+#            build/firefox/manifest.json  (dropped when Firefox restarts)
+#   Chrome   chrome://extensions -> Developer mode -> Load unpacked ->
+#            build/chrome/
 #
 # Publishing:
-#   Firefox  upload dist/*.xpi at addons.mozilla.org
-#   Chrome   upload dist/*-chrome.zip at chrome.google.com/webstore/devconsole
+#   Firefox  upload dist/*-firefox.xpi at addons.mozilla.org
+#   Chrome   upload dist/*-chrome.zip   at chrome.google.com/webstore/devconsole
+#
+# Every package filename names its browser. The two are not interchangeable --
+# they differ in the manifest's background key -- and uploading the wrong one
+# fails in ways that are not obvious from the error.
 
 EXT_ID := logsdu
 VERSION := $(shell node -p "require('./package.json').version")
+FIREFOX_DIR := build/firefox
+CHROME_DIR := build/chrome
 DIST := dist
-XPI := $(DIST)/$(EXT_ID)-$(VERSION).xpi
+XPI := $(DIST)/$(EXT_ID)-$(VERSION)-firefox.xpi
 CRX := $(DIST)/$(EXT_ID)-$(VERSION)-chrome.zip
 
-.PHONY: all build chrome test smoke xpi crx packages clean check-deps
+.PHONY: all firefox chrome typecheck test smoke xpi crx packages clean \
+	distclean check-deps
 
-all: build
+# The default does the lot: build both browsers and package both. Packaging is
+# only a zip of a directory that was going to be built anyway, so making it the
+# default costs nothing and means dist/ is never quietly out of date with src/.
+all: packages
 
 # Unit tests for the pure logic: the input formatters and the decision that
 # says whether a page load may press "Entrar". Run straight through Node's
@@ -64,30 +78,40 @@ test:
 	node --test "src/**/*.test.ts"
 
 # Runs the built background bundle in a service-worker-shaped sandbox, which
-# is where a Chrome-only breakage would otherwise hide until runtime.
-smoke: build
-	node tools/sw-smoke.mjs
+# is where a Chrome-only breakage would otherwise hide until runtime. The
+# bundle is identical for both targets, so checking one covers both.
+smoke: chrome
+	node tools/sw-smoke.mjs $(CHROME_DIR)/background.js
 
-# Call the local toolchain directly, so this works regardless of how pnpm is
+# Typecheck once. Both build targets depend on it rather than each running tsc,
+# which halves the work when building both.
+#
+# Calls the local toolchain directly, so this works regardless of how pnpm is
 # provided (corepack vs standalone). Run "corepack pnpm install" first.
-build: check-deps
+typecheck: check-deps
 	node_modules/.bin/tsc -noEmit -skipLibCheck
+
+firefox: typecheck
 	node esbuild.config.mjs production
+	@echo "Firefox build: $(CURDIR)/$(FIREFOX_DIR)"
 
 # The same sources with Chrome's manifest. Firefox and Chrome disagree on the
 # background key and on the gecko block, so the manifest is generated per
 # target rather than forked.
-chrome: check-deps
-	node_modules/.bin/tsc -noEmit -skipLibCheck
+chrome: typecheck
 	TARGET=chrome node esbuild.config.mjs production
-	@echo "Chrome build in build/ -- chrome://extensions -> Developer mode -> Load unpacked."
+	@echo
+	@echo "Chrome build: $(CURDIR)/$(CHROME_DIR)"
+	@echo "Load it with chrome://extensions -> Developer mode -> Load unpacked."
+	@echo "Select the folder itself; Chrome wants the directory holding manifest.json."
+	@echo
 
 # An .xpi is just a zip of the extension directory, with the manifest at the
 # top level rather than inside a wrapper folder.
-xpi: build
+xpi: firefox
 	@mkdir -p $(DIST)
 	rm -f $(XPI)
-	cd build && zip -qr ../$(XPI) .
+	cd $(FIREFOX_DIR) && zip -qr $(CURDIR)/$(XPI) .
 	@echo
 	@echo "Built: $(CURDIR)/$(XPI)"
 	@echo
@@ -108,18 +132,30 @@ xpi: build
 crx: chrome
 	@mkdir -p $(DIST)
 	rm -f $(CRX)
-	cd build && zip -qr ../$(CRX) .
+	cd $(CHROME_DIR) && zip -qr $(CURDIR)/$(CRX) .
 	@echo
 	@echo "Built: $(CURDIR)/$(CRX)"
 	@echo "Upload it at chrome.google.com/webstore/devconsole"
 	@echo
 
-# Both packages. The Firefox build runs last so build/ is left in the state
-# the development instructions assume.
-packages: crx xpi
+# Both packages. Order no longer matters: each target has its own directory.
+packages: xpi crx
 
+# Removes everything the build produces, including packages from earlier
+# versions, whose filenames carry their own version number and so are never
+# overwritten by a later build. dist/ is tracked in git, so a clean shows the
+# packages as deleted until the next "make packages" puts them back.
 clean:
 	rm -rf build $(DIST)
+	rm -f $(EXT_ID)-*.xpi $(EXT_ID)-*.zip
+	find . -name '*.map' -not -path './node_modules/*' -delete
+	@echo "Removed build/, $(DIST)/ and any stray packages."
+
+# Everything clean removes, plus the installed dependencies. Recover with
+# "corepack pnpm install" -- never with npm, see the note in README.md.
+distclean: clean
+	rm -rf node_modules
+	@echo "Removed node_modules/. Run: corepack pnpm install" 
 
 # Fail with a useful message rather than a confusing "tsc: not found".
 check-deps:
diff --git a/README.md b/README.md
index 15da879..264185c 100644
--- a/README.md
+++ b/README.md
@@ -19,7 +19,7 @@ Once it is published, from addons.mozilla.org. Until then, build it yourself:
 
 ```
 corepack pnpm install     # first time, or after a dependency change
-make xpi                  # typecheck, bundle, and package dist/logsdu-<version>.xpi
+make xpi                  # bundle and package dist/logsdu-<version>-firefox.xpi
 ```
 
 `make xpi` prints the full path of the file and how to install it. The package
@@ -119,12 +119,12 @@ check.
 ## Chrome
 
 ```
-make chrome               # unpacked Chrome build in build/
+make chrome               # unpacked Chrome build in build/chrome/
 make crx                  # package it as dist/logsdu-<version>-chrome.zip
 ```
 
-Load `build/` via `chrome://extensions` -> Developer mode -> **Load unpacked**,
-or upload the zip at
+Load `build/chrome/` via `chrome://extensions` -> Developer mode -> **Load
+unpacked** (select the folder itself), or upload the zip at
 [the Web Store dashboard](https://chrome.google.com/webstore/devconsole).
 
 One codebase, two manifests. Chrome MV3 requires a background *service worker*
@@ -156,21 +156,29 @@ script instead.
 
 ```
 corepack pnpm install
+make                      # everything: build and package both browsers
+make firefox              # build Firefox only -> build/firefox/
+make chrome               # build Chrome only  -> build/chrome/
 make test                 # unit tests
 make smoke                # background bundle under a service worker
-make                      # typecheck and bundle into build/
-make packages             # both publishable packages into dist/
-corepack pnpm run dev     # rebuild on change
-make clean
+corepack pnpm run dev     # rebuild Firefox on change
+make clean                # remove build/, dist/ and stray packages
+make distclean            # clean, plus node_modules/
 ```
 
-`build/` is the unpacked extension and is regenerated from scratch every time.
-`dist/` holds the packages meant to be published, and is kept separate so that
-packaging never tries to include its own output.
+A bare `make` typechecks once, bundles for both browsers into `build/`, and
+packages both into `dist/`. Packaging is only a zip of a directory that was
+going to be built anyway, so it costs nothing and keeps `dist/` from drifting
+out of step with the sources.
+
+Each browser gets its own directory under `build/`, so the two can coexist and
+neither is ever left stale by the other. `dist/` holds the packages meant to be
+published, and is kept out of `build/` so that packaging never tries to include
+its own output.
 
 While iterating, load the unpacked directory rather than reinstalling an `.xpi`
 each time: `about:debugging` -> **This Firefox** -> **Load Temporary Add-on** ->
-`build/manifest.json`, then press **Reload** there after each rebuild. That copy
+`build/firefox/manifest.json`, then press **Reload** there after each rebuild. That copy
 disappears on restart, which is the point -- it is for development, not daily
 use.
 
diff --git a/VERSION b/VERSION
index 0d91a54..9e11b32 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-0.3.0
+0.3.1
diff --git a/dist/logsdu-0.3.0-chrome.zip b/dist/logsdu-0.3.0-chrome.zip
deleted file mode 100644
index 19692ba..0000000
Binary files a/dist/logsdu-0.3.0-chrome.zip and /dev/null differ
diff --git a/dist/logsdu-0.3.0.xpi b/dist/logsdu-0.3.0.xpi
deleted file mode 100644
index 3583969..0000000
Binary files a/dist/logsdu-0.3.0.xpi and /dev/null differ
diff --git a/esbuild.config.mjs b/esbuild.config.mjs
index 7422852..0fc5881 100644
--- a/esbuild.config.mjs
+++ b/esbuild.config.mjs
@@ -35,12 +35,17 @@ const banner = `/*
 `;
 
 const prod = process.argv[2] === 'production';
-const outdir = 'build';
 
 // Target browser. Firefox and Chrome disagree on exactly one manifest key, so
 // the manifest is written per target rather than duplicated in the tree.
 const target = process.env.TARGET === 'chrome' ? 'chrome' : 'firefox';
 
+// Each target gets its own directory. They used to share one, and the result
+// was that whichever build ran last silently won: loading the other browser's
+// output then failed with a confusing manifest error. Separate directories
+// mean both can exist at once and neither can be stale by accident.
+const outdir = `build/${target}`;
+
 /**
  * Write the manifest for the target browser.
  *
@@ -60,8 +65,9 @@ async function writeManifest() {
 	);
 }
 
-// Everything that is not TypeScript is copied verbatim into build/, so that
-// the directory can be handed straight to about:debugging.
+// Everything that is not TypeScript is copied verbatim into the output, so
+// that the directory can be handed straight to about:debugging or to Chrome's
+// "Load unpacked".
 async function copyStatic() {
 	await mkdir(outdir, { recursive: true });
 	for (const name of await readdir('src')) {
@@ -76,7 +82,7 @@ async function copyStatic() {
 await copyStatic();
 
 // In watch mode the static files must follow every rebuild, otherwise editing
-// manifest.json or a .html file silently changes nothing in build/.
+// manifest.json or a .html file silently changes nothing in the output.
 const staticPlugin = {
 	name: 'copy-static',
 	setup(build) {
diff --git a/package.json b/package.json
index 5471603..cdfeb53 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
 	"name": "logsdu",
-	"version": "0.3.0",
+	"version": "0.3.1",
 	"description": "Browser extension that fills and submits a three-field academic portal login.",
 	"author": "Ruben Carlo Benante <rcb@beco.cc>",
 	"type": "module",
diff --git a/src/manifest.json b/src/manifest.json
index 0e34fa1..6a79832 100644
--- a/src/manifest.json
+++ b/src/manifest.json
@@ -1,7 +1,7 @@
 {
 	"manifest_version": 3,
 	"name": "logsdu",
-	"version": "0.3.0",
+	"version": "0.3.1",
 	"description": "Saves and fills logins that password managers cannot: registration number, date of birth and document number.",
 	"author": "Ruben Carlo Benante (Dr. Beco)",
 	"homepage_url": "https://code.beco.cc/beco/logsdu",
diff --git a/tools/sw-smoke.mjs b/tools/sw-smoke.mjs
index de67ed2..5ab6a0a 100644
--- a/tools/sw-smoke.mjs
+++ b/tools/sw-smoke.mjs
@@ -29,14 +29,15 @@
 //
 // This evaluates the built bundle in a worker-shaped sandbox with a stubbed
 // extension API, and asserts that it registers exactly one content script, for
-// exactly the configured origin. Run it against build/ after a build:
+// exactly the configured origin.
 //
 //   make smoke
+//   node tools/sw-smoke.mjs [path/to/background.js]
 
 import { readFileSync } from 'node:fs';
 import vm from 'node:vm';
 
-const BUNDLE = 'build/background.js';
+const BUNDLE = process.argv[2] ?? 'build/chrome/background.js';
 const CONFIGURED_ORIGIN = 'https://portal.example.br';
 
 const permissionChecks = [];