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!