Works very well, but, and this is a big but, you need to be aware of the fact that the language pack on its own is not sufficient for spell-checking. The dictionary needs to be installed as well. This is not clearly indicated on the downlooad page.

Doesn't work

Does not work.

Pretty cool

It doesn't work. The spelling always defaults to US English. If I had to rely on a spell-checking browser, I'd use Chrome or Edge!

English (GB) unavailable as an option for spell checking. Only English US. Using FF97

Does not work, not now, not ever.

Uses American Spellings not English.

Language settings in Firefox is a complete mess.

Does work but sometimes you need to google a bit

Firefox will not use it and states only US pack available for my locale. So uses US spellings. I want to use British English!