> 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.

where is this button?

Fox years with no changes, seemingly.

> The dominos pizza website (at least in Ireland) basically never works with Firefox.

Sounds more like a feature than a bug /s