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.
• 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

• https://support.mozilla.com/kb/Using+the+spell+checker

Thank you so, so, so much. You were dead on!!

This was driving me nuts. It seems the dictionary chosen was Russian which I wanted to install at one point but never thought I did. I have corrected it and now all is okay.

Cheers, Marc

Please click the Solved It button next to the answer that solved your Firefox support issue, it appears when you are logged in, so this thread gets marked as Solved to help other users who may have this same problem.

Thank you so much. I had the same problem, most notably in Yahoo mail so the:

 "You can enable or disable spell checking globally:
   Tools > Options > Advanced : General: Browsing: "Check my spelling as I type" 

was the answer for me! thanks