> In the last 10 years I've not come across any site that doesn't work with Firefox

I have. The dominos pizza website (at least in Ireland) basically never works with Firefox. I normally end up using Safari for that particular site.

Additionally, lots of stuff doesn't work when Advanced Tracking Protection is on, enough that if I have any issues my first step is disabling that.

Did you ever click 'report broken website' button? It's there specifically for those cases.