If it is indeed enabled you may not have an active dictionary, please do the following:

Right-click a web form on any web page. Pay close attention to the options that are displayed. If you see Add Dictionaries you have no dictionaries installed and should do so. This is usually the case if spell checking is not working if the option is enabled in the settings.

Select: Add dictionaries and Firefox will re-direct you to a page that will allow you to select a dictionary. This is added as an add-on. This should fix your spell check problem.

Solved thanks - should be part of the installation

FYI:
Not all locales come with a dictionary installed due to licensing reasons.
If you do not have the en-US locale then check that you have a dictionary installed (Tools > Add-ons > Extensions) and selected.

• http://kb.mozillazine.org/Dictionaries
• http://kb.mozillazine.org/Spell_checking