The spell checker dictionary in Firefox seems hellbent on being set to en_ZW (Zimbabwe-variety English), despite my locale being set to en_US. If I clear the spellchecker.dictionary key in Firefox and reload a page, it resets to en_ZW. I had to install the en_US dictionary from addons.mozilla.org (https://addons.mozilla.org/en-US/firefox/addon/3497) and then manually force the spellchecker.dictionary key to en_US in order to finally get US-style English supported by the spellchecker in Firefox. No other application I run has this problem - they all use the American/US dictionaries for spell checking without any additional confgiuration.
*** This bug has been marked as a duplicate of 439598 ***