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?

Prático.

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