web-ext-cli-mozilla

Overview

web-ext is Mozilla's reference CLI for WebExtension development. Per the mozilla/web-ext README, it bundles five subcommands: run, lint, sign, build, and docs. web-ext v8 added a dump-config subcommand and reworked the signing flow to use the AMO submission API by default (per the web-ext command reference).

This skill wraps web-ext for test runs. Composes with:

• manifest-v3-test-surface-reference for the manifest fields web-ext lint enforces.
• mv2-to-mv3-migration-test-checklist for migration-time lint rules.

For Playwright-driven MV3 popup / content-script fixtures see qa-modern-web/browser-extension-tests. That skill is Chromium-only and assumes the extension is built; web-ext is the lint+run+build+sign tool that produces the artifact and validates it pre-flight on either browser.

When to use

• Validating a manifest + sources against the AMO linter rules before submission.
• Driving a fresh Firefox temporary profile to test an extension end-to-end without manual install.
• Building a deterministic .zip for upload to AMO or for CI artefact archiving.
• Signing an extension for self-distribution (unlisted) or AMO listing (listed).
• Running the same source on chromium from one CLI for parity smoke-tests (per --target chromium in we-cmd).

Authoring

Install

Per we-readme, install globally or per-project:

# Global
npm install --global web-ext

# Per-project (preferred for CI determinism)
npm install --save-dev web-ext

Mozilla recommends "the current LTS (long term support) version of NodeJS" (per we-readme) - verify your CI image matches before asserting reproducibility.

Project layout assumed by web-ext

my-extension/
  manifest.json
  background.js
  content.js
  popup/
    popup.html
    popup.js
  icons/
  web-ext-config.cjs   # optional, see below

web-ext operates on the directory passed via --source-dir (or -s); per we-cmd, --source-dir is a global option - available to every subcommand - and --artifacts-dir (default ./web-ext-artifacts) controls where build / sign write zips.

Optional web-ext-config.cjs

A config file lets CI assert the same flags every run. Run web-ext dump-config (new in v8 per we-cmd) to print the resolved configuration as JSON for diffing.

// web-ext-config.cjs
module.exports = {
  sourceDir: './dist',
  artifactsDir: './build/artifacts',
  run: {
    firefox: 'nightly',
    startUrl: ['about:debugging#/runtime/this-firefox'],
  },
  build: {
    overwriteDest: true,
    filename: 'my-extension-{version}.zip',
  },
  lint: {
    warningsAsErrors: true,
    output: 'json',
  },
};

Running

Lint

web-ext lint wraps mozilla/addons-linter and emits AMO-compatible warnings. Per we-cmd the flag list is:

CI-friendly invocation:

web-ext lint \
  --source-dir ./dist \
  --output json \
  --pretty \
  --warnings-as-errors > lint-report.json

--warnings-as-errors is the right CI default - by we-cmd this escalates lint warnings (e.g., unsupported manifest fields, MV2-only APIs flagged for MV3) into a non-zero exit code.

Run (Firefox desktop)

Per we-cmd, web-ext run builds the extension and installs it into a fresh temporary Firefox profile, then watches the source directory for changes and reloads on edit:

web-ext run \
  --source-dir ./dist \
  --firefox=firefox \
  --start-url 'about:debugging#/runtime/this-firefox'

Firefox alias values accepted by --firefox (per we-cmd): firefox, beta, nightly, deved / firefoxdeveloperedition, or a full path to a Firefox binary.

To pin a profile (e.g., to retain auth state between runs):

web-ext run \
  --firefox-profile=qa-profile \
  --profile-create-if-missing \
  --keep-profile-changes

--keep-profile-changes (per we-cmd) persists profile modifications across runs - useful for snapshot-style tests.

Run on Chromium

Per we-cmd (the --target flag), web-ext run supports three targets: firefox-desktop, firefox-android, and chromium. To smoke-test cross-browser:

web-ext run \
  --source-dir ./dist \
  --target chromium \
  --chromium-binary "$(which chromium)"

This is the lowest-effort way to verify a Firefox-developed extension at least loads on Chromium without a full Playwright fixture. Deep Chromium-specific test surface lives in chrome-extension-test-loader and playwright-extension-fixtures.

Run on Firefox Android

web-ext run \
  --target firefox-android \
  --android-device emulator-5554 \
  --firefox-apk org.mozilla.firefox

Per we-cmd, the --adb-* family of flags wires adb for the device handshake (--adb-bin, --adb-port, --adb-host, --adb-device/--android-device, --adb-remove-old-artifacts).

Build

web-ext build \
  --source-dir ./dist \
  --artifacts-dir ./build/artifacts \
  --overwrite-dest \
  --filename 'my-extension-{version}.zip'

Per we-cmd, --filename / -n defaults to {name}-{version}.zip. --overwrite-dest / -o is required when the same artefact path already exists (e.g., re-running build in the same CI job).

Sign

Per we-cmd, web-ext sign v8 uses the AMO submission API by default; --channel is required.

export WEB_EXT_API_KEY='user:12345:1'
export WEB_EXT_API_SECRET='abcdef...'

web-ext sign \
  --source-dir ./dist \
  --channel listed \
  --amo-metadata ./amo-metadata.json \
  --upload-source-code ./source.tar.gz

Channel semantics (quoted from we-cmd):

"with listed the extension 'gets submitted for public listing'; with unlisted it 'gets submitted for signing for self-distribution.'"

To submit updates, per we-cmd the manifest must include an extension ID (browser_specific_settings.gecko.id for Firefox).

Parsing results

Lint output shape

With --output json --pretty, web-ext lint emits an addons-linter report:

{
  "count": 2,
  "summary": { "errors": 1, "notices": 0, "warnings": 1 },
  "metadata": { "manifestVersion": 3, "type": "extension", ... },
  "errors": [
    {
      "_type": "error",
      "code": "MANIFEST_FIELD_INVALID",
      "message": "...",
      "description": "...",
      "file": "manifest.json",
      "line": 12,
      "column": 5
    }
  ],
  "warnings": [...]
}

(Field shape per we-readme's reference to mozilla/addons-linter; exact fields stable across recent versions but spot-check the addons-linter changelog before pinning a parser.)

Parse with jq:

jq '.summary.errors + .summary.warnings' lint-report.json
# > 0 means CI should fail under --warnings-as-errors

Build output

web-ext build prints the artefact path to stdout and exits non-zero on lint failure (build runs lint first). Capture via:

artefact=$(web-ext build -s ./dist -a ./out --overwrite-dest \
  | grep 'Your web extension is ready' \
  | sed -E 's/.*: (.+)$/\1/')

Sign output

web-ext sign exits zero on signed-and-downloaded; non-zero on AMO rejection or timeout. Signed .xpi lands in --artifacts-dir.

CI integration

GitHub Actions example, gated lint + build on every PR + sign on tag:

name: extension-ci
on:
  pull_request:
  push:
    tags: ['v*']

jobs:
  lint-build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v5
      - uses: actions/setup-node@v5
        with: { node-version: 'lts/*' }
      - run: npm ci
      - name: Lint
        run: npx web-ext lint -s ./dist -o json --pretty -w > lint-report.json
      - name: Build
        run: npx web-ext build -s ./dist -a ./out --overwrite-dest
      - uses: actions/upload-artifact@v4
        with:
          name: lint+xpi
          path: |
            lint-report.json
            out/*.zip

  sign:
    if: startsWith(github.ref, 'refs/tags/v')
    needs: lint-build
    runs-on: ubuntu-latest
    env:
      WEB_EXT_API_KEY: ${{ secrets.AMO_JWT_ISSUER }}
      WEB_EXT_API_SECRET: ${{ secrets.AMO_JWT_SECRET }}
    steps:
      - uses: actions/checkout@v5
      - uses: actions/setup-node@v5
        with: { node-version: 'lts/*' }
      - run: npm ci
      - name: Sign (listed)
        run: |
          npx web-ext sign \
            -s ./dist \
            --channel listed \
            --amo-metadata ./amo-metadata.json

--warnings-as-errors in the lint step is what gates the PR - per we-cmd it converts warnings into exit-1.

Anti-patterns

Limitations

• Linter ≠ tests. web-ext lint catches manifest issues and AMO policy violations; it does not exercise the extension's behaviour. Pair with playwright-extension-fixtures for behavioural assertions.
• Chromium target is run-only. web-ext run --target chromium installs the unpacked extension into a fresh Chromium profile, but per we-cmd does not provide the AMO-style signing path - Chrome Web Store has its own publishing flow.
• No Firefox MV3 parity check. web-ext lint does not warn when a manifest uses a key that's MV3-supported in Chrome but not Firefox (or vice versa); cross-reference the manifest-v3-test-surface-reference matrix manually.
• Approval timeout caps signing throughput. --approval-timeout defaults to 900000 ms (15 min) per we-cmd; AMO can take longer in queue, requiring follow-up polling.
• Self-hosted mode disables AMO warnings. --self-hosted (per we-cmd) is for enterprise self-distribution; do not enable on AMO-bound builds.

References

• Mozilla web-ext README (install, basic usage) - github.com/mozilla/web-ext.
• Extension Workshop - web-ext v8 command reference (every flag on every subcommand) - we-cmd.
• Mozilla addons-linter (what web-ext lint wraps) - github.com/mozilla/addons-linter.
• Composes: manifest-v3-test-surface-reference.
• Siblings: chrome-extension-test-loader, playwright-extension-fixtures.