IIRC, Firefox's web extension API does not provide access to accessibility tree as well.

I'm not GP, but I agree that if your goal is to empower the end user and protect him from corporate overlords, then Firefox is a more logical choice to fork from.

I wonder if someone on the Chromium team will upstream all these BrowserOS changes, or "Not Invented Here" and re-implement it all for Gemini / Google Assistant.

Can't imagine Firefox acquiring a Firefox fork!

The hooks needed for integration will probably be implemented in Chrome, not Chromium. Nobody else should have them.

Isn't the Firefox code notoriously hard to fork and work with? I'm sure that nearly all of these Chrome forks would prefer to fork Firefox, but there's a reason they don't.

The reason they usually don't is for better compatibility with more web pages.