I did the following

time and date is correct firefox network no proxy uninstalled and reinstalled version 46.0.1 (still) i didn’t installed Nvidia Forceware Network Access Manager … don’t have Eset ,mbr and 360AV (still) clearing Firefox’s cookies and cache (still) Can’t sign into Firefox, getting “You must be connected to the Internet to sign in.” Disable Windows defender (still) Firefox is ticked on Firewall settings.

Every other browser and internet-capable application is working fine, What’s up with FIrefox ? I tried beta (latest 47) and stable.version which throw same error.

I’ve Adgaurd premium 6.0 installed though but uninstalling it doesn’t make any difference and I tried restarting my computer dozen time.

I can open non-http sites albeit not fully, like, It’s text/hyperlink only!

Your answer will be in this link I think. Adgaurd plus could have been corrupted somehow and didn’t fully uninstall and somehow broken your proxy settings in the registry. Just guessing. Good Luck.