Spell checking in web forms is very critical for me when deciding what browser I use the most and IESpell has been a solid choice for Internet Explorer users over the past couple years. To date, Firefox users have not had good spell checking options with each update or release to the browser breaking the previously installed spell checker.
We started with the "Firebird Spell Checker", then "Getting textarea spell checking to work with Firefox 0.8", and the ever complicated making it work in "Firefox 0.9" (which I never did get to work).
The solution: Spellbound Spell checker for Firefox 0.9+. Not only does this extension work great in 0.9 but it also comes with the simplest install yet compared to previous versions.