While looking for a way to spell check my blog posts I came across this very handy extension. SpellBound – Spellchecker for Firefox and the Mozilla Suite
I actualy found that following the install instructions didn’t work and after installing all 3 bits, I had to reinstall the spellbound extension to get it working. Alternatively, install the additional bits first, then the spellbound extension which seems to work too. YMMV.