Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

It is possible that (anti-virus) security software is causing the problem. Try to disable security software temporarily to see if that makes a difference.

Hi again, I tried stopping all the extensions but it did not solve the problem. My Theme is already in `default`. Here is some new clues I've collected about the issue;

I was watching a video on Youtube and in the middle of the video the playback stalled as if the connection was lost. I copied the url of the video and pasted it in a new tab and it worked fine on the new tab while it was stalled in the first one. This show that the loading problem doesn't only appear during the initial loading of a page. It can also happen 15 mins after opening a tab, during a video chunk download.

Another thing I've noticed is, the tab page becomes functional again if I open `about:config` page. This is the only thing I have discovered yet that recovers the tab from being non-responsive. (Typing in another url, using the Home button or a Bookmark doesn't help)

It looked like a disk I/O error that a browser cache chunk is randomly being written over a bad sector that Firefox can't read but I checked the SSD for errors and it's all OK.

What could be the culprit?