Not sure how so many in the thread are using FF problem-free? In the past few years I've encountered more and more sites that don't work properly with FF. Most of the issues are around modals that won't dismiss or even display at all. Other issues I can't identify, but the site becomes unresponsive or some content won't load. I think the Ticketmaster ticket selection page was the last one I couldn't use at all with FF. My cable company's site became Chromium only sometime this year and it still is.

Sometimes the cause is uBlock Origin, so I always try turning it off and refreshing. Rarely it's due to enhanced tracking protection. A few times I restarted FF in safe mode to rule out add-ins. It's always just FF.