I think the future is a rebase on Chromium. With so many companies (Brave, Google, Microsoft) each shipping their own integrations in Chromium, integrations will naturally become plug-in based (or at least, have a fairly stable API), allowing each vendor to opt in or out of each customization. Then, projects like Tor will move the TorBrowser to Chromium with their own implementations that are tracker free and one of them will eventually take the place of Firefox as the rallying point for the free software ideologues.
Which is a shame because I have a lot of sympathy for the free software ideologues. I'd like to win the browser fight for user-first free software ideology, not just corporate directed free software. But I no longer see Mozilla or Firefox as a banner under which that can be done. At the same time, I'm not sure how next free banner gets started.
I don't know about web features lagging, I feel that's also just chrome pushing web features before they're fully ratified, but maybe there's some where firefox really lags. It was a bit painful for my project that they've held back on the shared buffers thing, but I think that's going to be turned on soon again.
Besides webrender, they've been really working at performance the last couple years, there's performance improvements in nearly every release.
If they're not fuzzing their interpreters that is worrying. If they really are doing bad, surely someone's gonna come and do it for them right? Or do you think Firefox is so small right now hackers aren't even bothering?