When you right-click on the red-underlined word, does it suggest possible correct words?

If it does not suggest the word you expect, is that a word that you yourself added with 'Add to dictionary'? Those words are not suggested.

Make sure that you have selected the correct dictionary.

You can see which dictionary is selected if you right-click in a text area and open the Languages submenu.
Also make sure that [[X]"Check Spelling" in the right-click context menu has a tick.
You can also try to toggle the "Check Spelling" item off and on again.

See http://kb.mozillazine.org/Spell_checking and How do I use the Firefox spell checker?

See also http://kb.mozillazine.org/Dictionaries