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 sub menu. At least one dictionary should be installed and have a check mark to indicate that it is selected. *open the "Add Dictionaries" link to install a dictionary if you do not have one. The spelling checker is enabled if [X] "Check Spelling" in the right-click context menu has a checkmark. You can enable or disable spell checking globally: *Options/Preferences -> General: Language and Appearance: "Check your spelling as you type" You can look here for dictionaries: *https://addons.mozilla.org/firefox/language-tools/ Note that for spell check you need a dictionary. A language pack provides string translations for the user interface. *https://support.mozilla.org/en-US/kb/use-firefox-interface-other-languages-language-pack You can check on the about:config page that <b>layout.spellcheckDefault</b> is set to 1 (multi-line) or 2 (single-line. *https://support.mozilla.org/en-US/kb/about-config-editor-firefox