Do you happen to have any examples? I’m curious to see how broken/what the issues are.
Among them: Logging into some of my financial accounts doesn't work on Firefox. Enterprise software and gear like VMware and management UIs of various devices on the network. (They foolishly hard-coded their devices to reject any UserAgent strings that weren't Chrome, IE, or Edge.) Sites that use some kind of poorly-implemented tracking/fingerprinting to make sure you're a human. (I would routinely get stuck in infinite CAPCHA loops even on normal sites.) For a while, Slack video/audio calls did not work on Firefox because Slack chose to use codecs that FF didn't support. Video calls on FF are still hit-and-miss on various platforms, ran into it on Facebook just the other day.
These are all just off the top of my head, of course. There are plenty more that I've forgotten.
A few sites would silently break, e.g. restaurant online order pages, but work in Chrome. Never really looked into why, it was just annoying and intermittent (might work one month but not the next).
YouTube occasionally had some issues. For a while it was on an old version of Polymer that used Shadow DOM V0 (experimental) instead of V1.
A good list is here https://webcompat.com/issues?page=1&per_page=100&state=open&... keep in mind some of these are "is extremely slow in Firefox". Sometimes that's just that Firefox didn't have the same set of optimizations (not necessarily even fewer optimizations, just not ones built against) and other times that's deeper seated like the Shaw DOM V0 example where the fallback for the page was to use some older.