Can you please provide more information that it is happening on a specific website or everywhere. One of the reason might be that you would have opened several other programs and you pc might not be able to handle the load or it might be due to addons check it out by running the safe mode.

Hello Arth,

 Tried Safe Mode w addons disabled, etc. No change. Not running any other programs on the computer. Seems to hang (slow down) more with Facebook, Twitter and Gmail / Youtube. The browser does the slow-down and speed-up (and slow down again) on all sites though. Haven't visited any sites other than regular social media and regular e-mail. Wasn't having this issue until 3 weeks ago.

Is there anything else to try?

Boot the computer in Windows Safe Mode with network support to see if that has effect.

• https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• https://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

Will try and post results.

I also feel a slow down that appear randomly on any website, but I have this issue with all programs that use graphics accelerations on my 144hz monitor, not only Firefox.

I suspect the graphics card slows down its clocks after a while and I have to reduce and maximize the window to "wake it up" and get a smooth display again.

Can you confirm that you get the same symptoms?

Actually, the Firefox will be quite speedy for some time, usually a few hours or so. Yesterday it hardly slowed at all. Today, however, it did slow down noticeably, for a few minutes, then back to normal for about an hour, then slow again, then normal. It comes and goes. Still to try starting Windows in Safe Mode as per cor-el's suggestion, just as an experiment. I've had little experience with this stuff, so I'm taking things step by step.

Hello again,

 Ok - Here's an update: Been doing alot of research on the matter. Noticed I was getting a Windows Runtime Error that was pointing to a scanner program driver. I went into the Task Manager and clicked 'End Process' on this scanner program driver. For about 2 days the browser ran smoothly, and no more error messages from Windows. I thought it had fixed the issue.

 However, today, the symptoms have returned (speeding up and now spending more time being slow than before). I booted Windows 7 in Safe Mode with Networking as per cor-el's suggestion and the symptoms are still there, though they seem less severe (less time slow, more time fast, but still back and forth). 

 Any further suggestions as to how to fix this issue?