Where are the sites that don't work in Firefox?

The fundamental problems are that, (1) there as not been a meaningful "market" for web browsers for 25 years due to platform bundling, making all browsers loss-leaders that have to be sustained by other revenue sources, and (2) not controlling any end-user computing platform or hyper-popular web destinations, Mozilla does not have any "stickiness" to meaningfully change that, much less actually push for new functionality.

The final blow was when Microsoft ditched their in-house browser engine in favor of a Chrome reskin, replacing its many "Google tendrils" with even more "Microsoft tendrils".

Tendrils that are *overwhelmingly popular*, I might add -- After all, who doesn't want all their history and bookmarks and notifications synchronized between multiple systems? Who doesn't want to be able to watch Netflix or Disney+ content? Or have harmful/malicious content automatically blocked? And now a new wave of mothership-tied AI-driven features is landing in the commercial browsers, and it's yet another front that Mozilla has to fight uphill battles on... just to maintain (not even gain!) their paltry market share.

The fact of the matter is that users overwhelmingly want these large integrated platforms. The ones that don't, ie the supposed "faithful", were never more than a rounding error on top of a rounding error [1], continually decrying all of Mozilla's attempts to frantically keep up with their competitors (and occasionally try to get ahead with hail marys)

[1] Windows remains the overwhelmingly primary platform, and even on Linux, far more use Chrome[-ish] than Firefox[-ish], and of those, even fewer have ad/tracker blockers set up, and even fewer aren't perpetually signed into $mothership that is actively pushing $other_browser.