Some problems occur when your Internet security program was set to trust the previous version of Firefox, but no longer recognizes your updated version as trusted. Now how to fix the problem: To allow Firefox to connect to the Internet again;

• Make sure your Internet security software is up-to-date (i.e. you are running the latest version).
• Remove Firefox from your program's list of trusted or recognized programs. For detailed instructions, see

Configure firewalls so that Firefox can access the Internet. {web link}

Well it worked for some websites and not others. Of the ones I use regularly American Airlines will let me sign in but then reverts to a screen saying they are updating their website! I assume that probably means they have a problem with their own handshake with Firefox now although they deny anything but are quick to recommend just moving to Google Chrome (which does work).