Do you have a dictionary installed and selected? The spelling checker is enabled if [X] "Check Spelling" in the right-click context menu has a checkmark. 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. You can enable or disable spell checking globally: *Options/Preferences -> General: Browsing: "Check my spelling as I type" You can look here for dictionaries: *https://addons.mozilla.org/firefox/language-tools/ Note that you need a dictionary and not a language pack that provides string translations for the user interface.