I came across an awesome tweak for Firefox that enables spell-checking in one-line input boxes. Normally, spell-check only works on <textarea> elements, but with this tweak, you'll see the familiar red underline when you misspell words in <input type="text"> form elements:

1. Open a new tab in Firefox and type "about:config"
2. Change the value of "layout.spellcheckDefault" to "2" (without the quotes)

And that's it! You don't have to restart Firefox for the changes to take place, so feel free to keep browsing. This tweak works in Firefox 2.x and 3.x, though I've not been able to verify it for myself in Firefox 2 since I'm now running Firefox 3; if you're still using the 2.x version, let me know if this works for you!

Special thanks to Gina Trapani of Lifehacker for including this in her Power User's Guide to Firefox 3.