Author, lint, run, build, and sign a Firefox / Chromium WebExtension using Mozilla's `web-ext` CLI v8. Covers `web-ext lint` (addons-linter wrapper, JSON output for CI), `web-ext run` (temporary install in firefox-desktop / firefox-android / chromium targets with hot-reload), `web-ext build` (deterministic zip), and `web-ext sign` (AMO submission API, listed vs unlisted channels, JWT credentials). Use when the extension targets Firefox (signing is mandatory for distribution) or when cross-browser test runs need a single CLI that drives both Firefox and Chromium against the same source tree.