In Ubuntu there is a spellchecker somewhere which underlines incorrectly spelled words in red. I dont think its part of Firefox since it seems to do it more or less everywhere, but that doesnt really matter.

Its very handy since my spelling is awful, but unfortunately so is my mouse control today and I just hit the "add to dictionary" button when aiming for the "replace word" button. Does anyone know where the personal dictionary is kept for this spellchecker so I can remove the accidentally added spelling?

Just guessing...

In terminal :
cd ~/.aspell/
ls
http://aspell.sourceforge.net/man-html/index.html

As skidzilla mentioned, common apps often use aspell. It should be installed by default. Open synaptic and check: alt+f2, then "gksudo synaptic"

Firefox 2.0 has integrated spell checking. Edgy Eft or 10.06 should have 2.0 by default.

With Dapper you can just grab a tarball and decompress from the mozilla site. Run it from your /home or throw it into /usr/local with a symlink popped into /usr/bin. I didnt see it in backports, as I think that Ubuntu is so dependent on a particular version of Firefox that removing 1.5 would be a bad idea.

Theres always SpellBound. http://spellbound.sourceforge.net/index
Never used it.