I accidentally added a word to my Firefox dictionary today, and after a quick Google Search I found this article.
Basically you need to find the persdict.dat. It can be found here in the following operating systems:
Windows XP
C:\Documents and Settings[User Name]\Application Data\Mozilla\Firefox\Profiles\xxxxxxxx.default\
Windows Vista
C:\users[User Name]\AppData\Roaming\Mozilla\Firefox\Profiles\xxxxxxxx.default\
Mac OS X
~/Library/Application Support/Firefox/Profiles/xxxxxxxx.default/
Linux
~/.mozilla/firefox/xxxxxxxx.default/
After you have located persdict.dat, open it in your favorite text editor and simply remove the misspelled word.