Re: The problem
If you can't maintain something then you have drop it. Not so sure on what kind of UI stuff you really need for extensions – I've yet to come across an extension that only exists for Firefox that I need – but maybe following Vivaldi's lead there and switching to JS will be the way to go. Hell of a migration path but I suspect it could be partially automated. I hate JS but the toolchain is now pretty sophisticated and not having don't have to maintain your own multiplatform UI kit is a big win.
Vivaldi definitely, it's now my second browser, demonstrates that you can go beyond merely skinning Chrome.