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 ***