Not all locales come with a dictionary installed due to licensing reasons.<br /> If you do not have the en-US locale then check that you have a dictionary installed (Tools > Add-ons > Extensions) and selected. You can see which dictionary is selected if you right-click in a text area and open the Languages submenu.<br /> You can open the "Add Dictionaries" link to install a dictionary if you do not have one.<br /> Make sure that [X] "Check Spelling" in the right-click context menu has a tick.<br /> You can enable or disable spell checking globally: * Tools > Options > Advanced : General: Browsing: "Check my spelling as I type" See also: * https://support.mozilla.com/kb/Using+the+spell+checker You can look here for dictionaries: * https://addons.mozilla.org/firefox/language-tools/