"Firefox/Chrome duopoly"? You're forgetting Safari; they have larger market share than Firefox.
That said, I'm not really holding my breath for FF to grow again.
[1] https://www.reddit.com/r/browsers/comments/nkzrj0/comment/gz...
In about 6 months, I won't consider Google either. I'll simply start targeting reasonable, good, healthy Web standards and implement them with progressive enhancement.
If they support the standards, then there won't be a problem.
The Web is fragmenting and it's gonna be ugly.
As a result, Firefox extensions got dumber. Bye bye Vimperator, DownThemAll, etc.