Firefox's spell check defaulting (and reverting) to the wrong language

I'm using Firefox (v. 26 for Ubuntu). I'm in the U.S., and I keep setting the spell check to U.S. English (right click on a word marked as incorrectly spelled, choose Languages, then the correct one). It briefly remembers, but after an hour or so (even within the same session) it reverts to Canadian English.

I'm a web developer and am fine with hacking about:config, but although I've set spellchecker.dictionary to en_US, it doesn't seem to stick around.

Is there any other setting I'm missing?

Check to see if you have an "hunspell-en" package installed. Firefox will make use of any Hunspell dictionaries that are installed. These dictionaries are also used by LibreOffice however, so the solution is not as simple as just removing the package. Firefox looks in /usr/share/hunspell for dictionaries, so any ones that you do not want Firefox to use will need to be removed from there. On my laptop (running Arch Linux), most of the dictionaries in /usr/share/hunspell were just symlinks, so they can be safely removed without actually removing the dictionary itself.

However, keep in mind that this is not a permanent solution, since an upgrade to the package will just put those files/symlinks back into place.

Ultimately, I solved the problem by installing the hunspell-en-base package from the AUR, which is a stripped-down package containing just the en_US dictionary.

Firefox seems to randomly use any one of the installed dictionaries. Several English dictionaries get installed by default in Ubuntu. The solution is to remove all but the US English dictionary using apt-get.

From How can I change Firefox's default dictionary?, Insperatus posted this useful command:

sudo apt-get remove myspell-en-au myspell-en-gb myspell-en-za hunspell-en-ca && sudo apt-get install myspell-en-us

Before:

After: