I assume there’s no context menu API (yet?).
Correct; context menus are currently not supported.
But testing has revealed some layout differences.
That sounds about right. We worked pretty closely with the developers of the extensions currently enabled on release to integrate their extension’s UX with that of Fenix. I’m currently working on making those recommendations more broadly available on Firefox Extension Workshop, but it’s going to take some time to build that documentation.
Curiously, I see the same differences when I run it in Chromium Edge! I wonder why?
That’s coincidental but interesting.