You can see which dictionary is selected if you right-click in a text area and open the ''Languages'' submenu. Also make sure that [[X]"Check Spelling" in the right-click context menu has a tick. You can also try to toggle the "Check Spelling" item off and on again. See http://kb.mozillazine.org/Spell_checking and [[Using the spell checker]] See also http://kb.mozillazine.org/Dictionaries Some have reported that toggling the "Check Spelling" check-mark in the right-click context menu works. .......... See also: Tools > Options > Advanced : General: Browsing: "Check my spelling as I type" http://kb.mozillazine.org/layout.spellcheckDefault http://kb.mozillazine.org/spellchecker.dictionary ---- Verify on the about:config page that the pref ui.SpellCheckerUnderlineStyle is not set to "0" See http://kb.mozillazine.org/ui.SpellCheckerUnderlineStyle To open the ''about:config'' page, type '''about:config''' in the location (address) bar and press the Enter key, just like you type the url of a website to open a website. If you see a warning then you can confirm that you want to access that page.

I have the same issue as described. This started when I installed FF 3.6.6 - FF 3.6.5 works fine. Is it possible issue with this version?