Do you have the Dutch dictionary installed and selected? * https://addons.mozilla.org/firefox/addon/woordenboek-nederlands/ * You can see which dictionary is selected if you right-click in a text area and open the Languages submenu. * 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: * Tools > Options > Advanced : General: Browsing: "Check my spelling as I type" You can look here for dictionaries: * https://addons.mozilla.org/firefox/language-tools/ See also: * http://kb.mozillazine.org/Spell_checking * http://kb.mozillazine.org/Dictionaries