It is possible that you confused dictionary with language pack and that you actually do not have a dictionary installed. 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: *Settings -> General: Language and Appearance: "Check your spelling as you type" *https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker 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-another-language