Spell Check in FireFox

Google site editor does not have spell check. In Firefox, there is spell checker built in. But it does not work for me when I edited my Google sites.

According to Firefox spell checker statement:
 "Firefox automatically checks the spelling of words that you enter in text boxes containing more than one line. As soon as you finish typing a word, it is checked against the words in the installed dictionary."

I realized that maybe my dictionary is empty.
So I installed English United States (en-US) spell-checking dictionary in Firefox from :
https://addons.mozilla.org/en-US/firefox/addon/united-states-english-spellche/

 After I restart the Firefox, the spell checker works fine during my on line editing. To enable or disable spell checker, right click mouse and check or un-check spell checker.