How do I migrate an existing Chrome-only extension to extension.js for cross-browser support?

[Alptoga]

I already have an extension that works only in Chrome.
It uses manifest v3, a background service worker, and some Chrome APIs.

I want to make it work on Firefox and Edge too, but every browser seems to need small changes.
For example, Firefox does not support some chrome.* APIs, and Edge has its own store rules.

Can I use Extension.js to make my code run on all browsers without rewriting everything?
What is the best way to start? Should I move files into a new Extension.js project or can I migrate step by step?

If anyone already did this, please share what worked for you.

I’m looking for tips about:

• updating manifest files
• fixing browser-specific APIs
• testing in multiple browsers before publishing

Thanks!

[cezaraugusto]

yep you can absolutely do this with Extension.js, and you can do it step by step (no need to rewrite everything at once)

if your extension already runs in Chrome (MV3 + service worker + chrome.*), the easiest migration path is:

1. add Extension.js to your existing project
2. wire scripts like:
   • extension dev --browser=chrome (since v3 we default to Chrommium)
   • extension dev --browser=edge
   • extension dev --browser=firefox
   • extension build --browser=all
3. keep Chrome working first, then fix Firefox/Edge differences incrementally

for manifest updates,best approach is to use browser-scoped manifest fields instead of hand-maintaining multiple full manifests so you can keep Chromium settings under chromium:* (or chrome:* / edge:*) and Firefox-specific behavior under firefox:*. Extension.js will normalize/build per browser target.

for browser-specific APIs, you’ll still need small compatibility fixes, but you don’t need to fork the whole codebase. what usually works best:

• use browser.* style APIs + webextension-polyfill for Chromium targets
• keep browser-specific branches minimal and feature-detect when possible
• only special-case behavior where parity is truly missing

for a multi-browser testing workflow, before publishing, run each browser directly in dev mode:

• extension dev --browser=chrome
• extension dev --browser=edge
• extension dev --browser=firefox

then produce all bundles with:

• extension build --browser=chrome
• extension build --browser=edge
• extension build --browser=firefox

you’ll get separate outputs (ex: dist/chrome, dist/edge, dist/firefox) so you can smoke-test each package before store upload. let me know if you run into any issues!