I'll be typing in an input box online and notice that the spell checker is highlight nearly every word. I right-click and look at "languages" to find that firefox has started using the Dutch dictionary (which I installed because I VERY OCCASIONALLY need it). I reset the language to English. Next time I open Firefox, I will need to repeat this process. I can't find any instructions about setting the preferred dictionary, and it looks to me like firefox is just picking the first one on an alphabetical list.