I have received the same message and have been trying for 6 hours to sort it out with no success - the support system is rubbish. I have been unable to access my work emails since I downloaded the new version of firefox I can't even find out how to go back to the previous version instead of this one as I had no problems getting into my email account then

Hi, Worked it out! Go to: https://addons.mozilla.org/en-US/firefox/addon/remote-xul-manager/ Then "+Add to Firefox" Click "Install now" Then "Restart Firefox" Then click on 'firefox' on top left hand side then "Remote XUL Manager" A little window will pop up and click "add" then type in your email provider for example mine is "webmail.adam.com.au"

Hope this helps!