This is NOT British English.
realise or realize? British is realise, American is realize. So why does it say realise is wrong? And there are a bunch of other British words labeled as wrong.

Obwohl ich 2-sprachig bin, hilft es mir, Schreibfehler zu verhindern

Not compatible?

just great

This pack is broken, it will not work on new versions of Firefox.

You need to instead use: https://addons.mozilla.org/en-GB/firefox/addon/british-english-dictionary-gb/

Installed but nothing changes and only en-us is available

Since update does not seem to be working.