> I surely hope I’m wrong about this, but I fear I’m not.
I suspect that the remaining Firefox users are likely to be quite "sticky". If there are websites that don't work in Firefox (but work in Chrome and Safari), I haven't yet blundered into one. My guess is that those would be mainly corporate intranet sites/apps, which I don't have to deal with.
No platform deploys Firefox by default; you have to deliberately choose it. That is NOT true of Chrome or Edge. I doubt anyone has ever deliberately chosen Edge; you can only get Edge on Windows, where it's the default anyway. And the only people I know that have deliberately chosen Chrome over the OS default are developers.