Do you have a dictionary installed and selected? *You can see which dictionary is selected if you right-click in a text area and open the Languages submenu. * Open the "Add Dictionaries" link to install a dictionary if you do not have one. * Make sure that [X] "Check Spelling" in the right-click context menu is check-marked. You can enable or disable spell checking globally: * Tools > Options > Advanced : General: Browsing: "Check my spelling as I type" You can look here for dictionaries: * https://addons.mozilla.org/firefox/language-tools/ See also: * http://kb.mozillazine.org/Spell_checking * http://kb.mozillazine.org/Dictionaries * https://support.mozilla.com/kb/Using+the+spell+checker

If you are using a dictionary extension make sure it is up to date version as you may be using a older version that was compatible with older Firefox versions at time.