Removing a Misspelled Word From Your Firefox Dictionary
So I accidentally added a misspelled word to the dictionary in Firefox. I wanted to remove it so I looked through the preferences and I even checked out about:config, but no luck. So I did a little research and found that in Linux, Firefox settings are kept in a hidden file in your home folder (like most applications) and your custom dictionary words are kept in a file in that folder.
The hidden folder is called .mozilla and the path to the custom dictionary file is: ~/.mozilla/firefox/xxxxxxxx.default/persdict.dat
The x’s represent the name of the Firefox profile and if you don’t know what a Firefox profile is, chances are you only have one and it will be the only folder inside of the firefox folder.
One thing to remember is that if you just made the mistake you have to close Firefox completely for it to save the new settings to disk. So the misspelled word just sits in memory and you wont find it in the persdict.dat dictionary file until after Firefox has been closed out.
For Windows and Mac the process is very similar and is covered well in this Lifehacker article.