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 submenu.<br /> 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. *Make sure that [X] "Check Spelling" in the right-click context menu is check-marked. You can enable or disable spell checking globally: *Edit > Preferences > Advanced : General: Browsing: "Check my spelling as I type" You can look here for dictionaries: *https://addons.mozilla.org/firefox/language-tools/