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}

Thanks for your reply I will try it but the problem is not permanent but it just happens every 3-4 days. When it won't open with Mozilla web page, I open it with Chrome or Opera without any problems, Mozilla gives me a message to check my internet connection, really weird

Best regards