How to use American English spelling dictionary in Firefox?

My Firefox spellchecker was complaining this morning that I spelled 'neighbor' in the American English style, not the British English style ('neighbour'). Same is true for color (colour), analyze (analyse), etc. I've checked in the edit->preferences->content->language tab, and en-us is selected. I also found this link here:

http://ubuntuforums.org/showthread.php?t=1013043

Suggesting that there's some kind of system panel I can use to ensure that I've got the right language, but I can't see where that is (I guess that's for an older Ubuntu that let people get to system settings).

So either the dictionary for Firefox for en-us is corrupted, just a copy of the British English dictionary, or somehow the setting isn't propagated properly. How can I get the American dictionary back?

1. In a text field, open a context menu by pressing the right mousebutton.
2. Select Check Spelling if needed
3. In the Languages menu, select English / United States

If this did not work, (re-)install the hunspell-en-us package.

For Firefox, put "about:config" in the address bar and go there. Search for "spell", and find the variable "spellchecker.dictionary". If its value is something other than "en_US" (mine said "en_AU"), right click the entry and modify the value to "en_US".