Does the problem occur on a new profile?

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

The easiest solution would be to uninstall Firefox, use something like CCleaner to remove any leftovers of the browser, then reinstalling it. Let me know how this works out.

>>22177
Well, seeing as all the configuration information is stored in the profile, there's literally zero advantage to uninstalling Firefox. Uninstalling leaves the profile behind. You can just wipe the profile with it still installed and get to the same point.

But when you wipe it you end up losing all the information in it. By testing with a new profile, leaving the old profile intact, you verify whether the problem occurs under the new profile before doing anything to your saved data under the old profile.

If the new profile has the same problem, then you know it's not anything in Firefox or the new profile, it's something else in the system. Typically this means some kind of proxy server getting in the way, which are annoyingly bundled with antivirus/internet security now. Either disable or uninstall the proxy and your problems should go away.

Of course there's also malware out there that proxies information too and they're typically not as well written as the shitty-ass proxy servers included with AV software.