For Firefox 2.0 Users Only

I recently upgraded my browser to Firefox 2.0, which I was happy to discover solved the one irritating feature of Firefox--the inability to select text from a web page and paste it into an Outlook email. However, 2.0 comes with a new annoying feature--automatic spell-checking--with no documented way to turn it off. If, like me, you can spell just fine, thank you, or if you can't spell to save your life but you just don't care, here's what you need to do to disable this feature:
In the browser address bar type
about:config

(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
layout

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.