Firefox now has a built spell checker. Works pretty good with the forum.
Dictionary Tooltip -- a Firefox add on does not work while typing but pops up a choice of detailed dictionaries and links to Wikipedia and others when you doubleclick a word you don't know.
https://addons.mozilla.org/en-US/firefox/addon/1171
Tinyspell is a great little spell checker that checks all you type and offers correct spellings.
http://tinyspell.m6.net/