Been using Firefox forever, since it was called Firebird, migrated to it from Netscape at some point.
One example of a page that doesn't work is Amazon's "Today's Deals"[1] page.
[2] is screenshots of the same page in Firefox (top) and Chrome (bottom). I do have uBlock Origin installed but disabled for Amazon.
I think Firefox's best shot for relevance is to switch from being a complete browser codebase and into being a maintained patch set for Chromium that retains user-focused features (such as adblock APIs and limits on media autoplay) as they are removed by Google. The required resource commitment would be far lower and might be viable in the long run.
That ship has already sailed.