Type '''about:preferences#advanced'''<Enter> in the address bar. Under '''Advanced,''' Select '''Network.''' Look for '''Configure How Firefox Connects''' and press the '''Settings''' button. If you are using a proxy, make sure those settings are correct. If there is no proxy, first use '''No Proxy.''' If there is a problem, then try '''System Proxy.''' ------------ Some problems occurs 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 '''[https://support.mozilla.org/en-US/kb/configure-firewalls-so-firefox-can-access-internet Configure firewalls so that Firefox can access the Internet.]''' {web link}

In some cases, you need to do a reset to the Windows TCP components. See: [https://support.mozilla.org/questions/1075112 After upgrade to window 10, cannot access internet from firefox or chrome].