Recently went from FireFox 2.0 to 3.0...could be the reason? They have a "Check Spelling" icon in the tool bar, but this isn't automatically activated like the forums previous underlining method.
I knew harmony was only a click away, thanks Gnomey!
Clicking right I didn't find "check this field" but there was "Add Dictionaries". This takes you to Firefox Add-ons, and it's restored when this is downloaded.