Use Mozilla's web-ext CLI to load and auto-reload a Firefox extension for manual and automated testing
domain: extensionworkshop.com · 5 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
Install with npm install --global web-ext (Node LTS recommended), or as a project devDependency
From the extension's source directory where manifest.json lives, run web-ext run to launch Firefox with the extension temporarily installed
Use --source-dir to point at the extension source when running from elsewhere, and --firefox to target a specific binary/channel such as Firefox Developer Edition
Leave web-ext run active during development — it watches source files and reloads the extension automatically on change, and pressing r in the terminal forces a manual reload
For CI smoke tests, pair web-ext lint for manifest/static validation with a scripted web-ext run session driven by a WebDriver client for real interaction testing, since web-ext itself doesn't run test assertions
Known gotchas
The extension installed by web-ext run is temporary and is automatically removed when that Firefox instance closes, so it can't test permanent-install-only behaviors like update flows without a separately signed/packaged build
web-ext run uses a fresh temporary Firefox profile by default, so tests relying on existing browser state (saved logins, other extensions) need --firefox-profile pointed at a real profile explicitly
web-ext lint's validation rules track Firefox's current manifest support and can flag a manifest key as invalid even though it's valid for Chrome, since MV3 support details differ between browsers
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