Typos often happen, even to the best of typists. Unfortunately these typos can be added to the spell check with a simple clicko as you attempt to correct your typo. Fortunately, its relatively easy to fix this mistake, even if it is a bit of a pain.
You will have to edit a file called
persdict.dat. This file is found in the folder for your Firefox profile, which in Linux is located in
messofletters with the actual name of the directory in ~/.mozilla/firefox/ that ends with
Having opened up
persdict.dat for editing, simply find and delete the typo you added, and then, having saved the file, restart Firefox.
This post (sort of) via Lifehacker.