A few days ago there was a Firefox update and this morning suddenly GeckoMain started (out of nowhere) using the CPU at 80% (which lead to the CPU fan going nuts). I opened the gnome system monitor and it showed that GeckoMain was overusing the CPU. There's also a process running called "Privileged Content". For as long as this additional process is running, Firefox is behaving as if there were no internet access - infinite time to load any page and if left undisturbed, after a while Firefox closes with a crash message. Also, for as long as this problem is happening, simply closing Firefox with the X button doesn't close it - GeckoMain is still running and the only way to actually close the browser is "killall GeckoMain". So I removed all extensions one by one in order to make sure it's not an extension causing this. It's not. Even after removing all extensions the problem continued which lead me to believe something in my prefs.js is causing it. There were so many things modified in about:config over the years that I have no idea which one might be causing the problem. I have uploaded the prefs.js file here, so that you can review it, if needed: https://drive.google.com/file/d/1j-EAaeBrcawRfzrIr4n9Pg6IyvM9zROV/view?usp=sharing At the moment I'm writing this question through Waterfox bc Firefox is unuseable with this problem.