Hi It seems that you installed the en-GB language pack and not the en-GB dictionary. This is a misunderstanding we regularly see happen. Dictionaries would appear under "Options/Preferences -> Dictionaries" You can look here for dictionaries, so make sure to look under the correct column: *https://addons.mozilla.org/firefox/language-tools/ For spell checking you need a dictionary. *https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker A language pack provides string translations for the user interface. *https://support.mozilla.org/en-US/kb/use-firefox-another-language 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.