Where are the sites that don't work in Firefox?

Then regulations should say that *blocking* minority browsers is called "discrimination", preferably under one of the disability acts (if I need a word-based browser to read to me, I should be able to use the browser that suits me, not them!).

Some form of pop-up (along the lines of that cookie thing) that says "you are using an unsupported browser" is fine, if objectionable.

Cheers,
Wol