Ubuntu

Comment 12 for bug 320266

Inside firefox type this into the location ("URL") bar: about:config
(no spaces around the ":" in "about:config")

A warning screen will pop up. Click whatever it takes to get past it.

Find the text field called "Filter" and type "spell" into it to narrow the list to just spell checker items.

Edit "spellchecker.dictionary" setting the string to your locale.

Pages related to this issue:

http://ubuntuforums.org/showthread.php?p=8523802

https://bugs.launchpad.net/ubuntu/+source/firefox-3.0/+bug/307600