Welp, Mozilla's trying to do cryptocurrency now (heads up: twitter link,) I've kinda just been waiting for the inevitable excuse to move on from Firefox.
Update: Nobody liked this and Mozilla stepped back on it because "we are reviewing if and how our current policy on cryto[currency] donations fit with our climate goals, but this was just an excuse for me to jump anyway.
Why? The writing's kinda been on the wall for Mozilla for a decade at this point. They've been losing users for years, going from 30% 10 years ago to, by most estimates, about 3% now. Arguably they were doomed as soon as Google came out with Chrome and started bundling it with all of their services. It got worse as the domination of Android happened, meaning everyone needed a Google account anyway and so using Chrome to take advantage of it was a no-brainer. It was never going to be possible for Mozilla to compete with Chrome, but it was important that Mozilla stuck around to prevent any anti-trust concerns; Mozilla was in effect paid to lose. Mozilla couldn't afford to develop Firefox fast enough because Google et al heaped so much garbage onto the web that you needed an unlimited budget in order to develop a browser. Additionally, since Microsoft's adoption of Google's rendering engine and the massive growth seen by Microsoft Edge (through their own monopolistic practices, but still,) I'm not convinced that Google needs to pay Mozilla to lose anymore. Google and Microsoft get a nice desktop browser duopoly. Google and Apple get their duopoly on mobile, everyone's happy. Mozilla tried for alternate funding sources,and so far this has pretty much been a miserable failure.
That's not to say that as an organization it was blameless. There were a lot of (imo) poor technical decisions, like fundamentally rewriting the browser to try and compete with Chrome on Chrome's strong points, where it had a decade's lead on them, rather than focusing on Firefox's strengths. They fired their R&D team for their new rendering engine features. They had this crazy idea for something called "chromeless" a decade ago which they unceremoniously killed: you may know the exact same idea now as a little thing called "Electron." A long time ago, it was possible to use their rendering engine in "native" applications, such as GTK+ on Linux or Win32 on Windows; I know because I wrote a browser using it. This was poorly supported and eventually dropped. A bunch of tiny web browsers switched to Webkit, then to Blink (Chrome's engine.)
Update: Day 2 and I remembered why the last dip into seamonkey failed: I can't actually do everything in it. Specifically I wasn't able to pay my power bill; had to boot up Firefox for that. I think at this point Blink and Webkit are sufficiently diverged that I'm comfortable trying to find a webkit browser to use, though. I figure the important sites will support "safari" even if they don't do Seamonkey.