This problem seems to be extremely common. And I am not sure there is any solution out there. But it is extremely annoying and I will definitely stop using Firefox if I can't get it fixed. I just switched to Ubuntu and I am having the most annoying problem with Firefox. It keeps defaulting to the wrong language for the Spellcheck dictionary. I have installed multiple dictionaries, but for some reason it keeps defaulting to Jordanian Arabic every time I open Firefox. I would like to make US English the default language. Changing the language in the contextual menu does not work, as it will just go back next time I open another tab or window. Changing the spellcheck dictionary in about:config is useless too because it will just change the next time I reopen FIrefox. Like I said, this bug is extremely annoying and if I keep having to deal with getting red squiggly lines under every single word I type, I will stop using Firefox and find a better browser.