Recently there was post in Go Firefox! in which the user had installed the British English Dictionary add-on but the spellchecker was still using the US English dictionary. There is an additional step once you have installed the new dictionary add-on and restarted Firefox. You need to tell Firefox which dictionary it should be using. While, you can install as many different dictionaries as you like (as seen in the example below), you can only use one at a given time. Right-click in a text-area Select Languages Select the Dictionary you want to use