Okay I feel really stupid posting this at all but I’ve struggled for half an hour to work out how to solve this problem to no avail.
I recently installed an updated version of the Firefox dictionary (GB english version) but now the spell checker seems to have become American as it’s flagging all my British spelt words as errors and gives me American versions instead.
I can’t figure out why that’s happened or, indeed, how to even select which dictionary I’m using (it’s not in the options section) nor does looking at the add on suggest anything. The help file is similarly unhelpful and google has failed me.
In your address bar, type in about:config and hit Enter.
You will be presented with a long list of configurable entries. Scroll down to the category spellchecker.dictionary.
The right-hand column, titled Value, should read en-US. If it doesn’t, right-click the entry, select Modify, then enter en-US in the Enter string value dialog box. Click OK.
That should do it, although you might have to restart Firefox in order for it to recognize the new setting.
In your URL bar, enter “about:config” (without quotes). Scroll down until you see the entry for spellchecker.dictionary. Double-click that entry and change the value to “en - GB”. Then restart Firefox.
Mine is the same and about:config is en-GB
Language on the list is British English.
It has always wanted to change words to the American spelling since day 1.