Comment 1 for bug 61326

Similar problem with Canadian users (en_CA): it seems that we also get the US english spell checker instead of British English. In Firefox, when looking at Help-->About, it shows that the version of Firefox installed is "en-US" (at least on my system).

If you download Firefox 2 beta manually from mozilla.org (http://www.mozilla.org/projects/bonecho/all-beta.html), you can select which language you want for your download, and choose between english and english (british). I assume that will install the spell checker in the language that you want.