Whoopee I've found the solution for FireFox, but it does mean going into the FireFox configuration file to change the default set-up and a few other tweaks. Follow the following steps copied from :

FireFox Spell Checker Web page default change

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 (where the http: etc would be) and press enter.
Now in the filter field, type the following value- layout.spellcheckDefault

(I went down the file listing to find this entry and changed it. Note that when you type in about:config as suggested a warning message is seen before you can proceed)



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).

Hope this helps, I also deleted the US versions of spell checker in the FireFox directory just to force the English/United Kingdom default. Using this option and with the spell checker ticked any options/suggestions to the words are highlighted at the top of the menu when "right button clicked" - simply select the correct one offered ie. favour instead of the now highlighted favor in the case of the UK dictionary set as default.

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BobGreg