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 You can look here for dictionaries: *https://addons.mozilla.org/firefox/language-tools/ 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. It is possible that a website has disabled the Firefox spell checker by adding a spellcheck="false" attribute to the input field. You can check this in the Inspector via the right-click context menu. *https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/ If that is the case then you need to enable the spell checker yourself.