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.<br /> You can open the "Add Dictionaries" link to install a dictionary if you do not have one.<br /> Make sure that [X] "Check Spelling" in the right-click context menu has a tick.<br /> You can enable or disable spell checking globally: * Tools > Options > Advanced : General: Browsing: "Check my spelling as I type" See also: * http://kb.mozillazine.org/Spell_checking * http://kb.mozillazine.org/Dictionaries * [[Using the spell checker]] You can look here for dictionaries: * https://addons.mozilla.org/firefox/language-tools/