For Firefox 2.0 Users Only
In the browser address bar type
(Note that there is no http:// preceding the command.)You'll see a whole host of registry-type settings for your browser.
In the Filter bar type
Scroll down to the value "layout.spellcheckDefault" and right-click on it. Select "Modify", and change the value to 0. You're done. No more annoying red squiggly lines under the words the browser doesn't recognize.
Lifehacker writes about a few more things here that you can do to tweak your browser just-so.