If you were able to load websites until you updated Firefox or until your Internet security software was updated, reconfigured or a new one was added, your Internet security software (including firewalls, antivirus programs, anti-spyware programs, and more) is likely preventing Firefox from connecting to the Internet.

or otherwise first clear your Firefox cookies and cache:

   Click the menu button New Fx Menu , choose History and select Clear Recent History....
   In the Time Range to clear: drop-down, select Everything.
   Click the arrow next to Details to display the list of items that can be cleared.
   Select both Cookies and Cache.
   Click Clear Now.

I did all of this but the problem still not solved yet. Thanks anyway.

probably a web site or user panel is not compatible with the version of Firefox.