Firefox has its own in-built spell check, but this spell check is available only in Text area and text fields (text boxes) will not show spelling mistakes. Many times blogging titles have spelling mistakes because of this. But a simple trick can activate spell check in text fields as well. To do this, open the config file of Firefox. Type about:config in address bar and press enter.
Now in the filter field, type the following value- layout.spellcheckDefault
The default value of this parameter is 1. Double click on this parameter and change the value to 2. Close config file and now open any page which has a textbox and check for wrong spelling (no restart required).
NB: In case the spell check is still not enabled, right click on the textbox and enable spell check.