If the entire browser is unresponsive to the point where you have to kill it via the task manager, it could be an issue with a memory leak. This page has some general information on high resource usage that might be helpful: Firefox uses too much memory or CPU resources - How to fix

Also, if there's a specific webpage or pages that always cause the problem to occur after 15 minutes or so, it might be worth trying to open those pages when in Troubleshoot Mode to see if you can get the same problem with Firefox "freezing" to occur when in Troubleshoot Mode. That'll confirm for sure that it's not an extension or other customization that's causing the issue. Here's how to enable Troubleshoot Mode: Diagnose Firefox issues using Troubleshoot Mode

You said that you have to use the task manager to kill Firefox — is the task manager showing an excessively high usage of memory or CPU by Firefox when this is going on, and does the Firefox task manager show the same high usage? It might be worth taking a screenshot of both Windows' task manager and Firefox's task manager when this issue is occuring, if you can, to confirm whether the resource usage is abnormal.

Here's how to access Firefox's task manager (this is different from Windows' system task manager) Task Manager - see whether tabs or extensions are slowing down Firefox And here's how to take a screenshot in Windows: How do I create a screenshot of my problem?

Hello Support, Some good news: when using Firefox in Troubleshooting Mode (for a week now) there is no 'freezing'.

However when using Firefox in normal mode, it does still 'freeze' after ten minutes approximately. In both modes I have added Firefox Task Manager (TM) on another tab and checked it frequently and it shows no signs of any problem. When the 'freezing' does occur I am unable to select the Firefox Task Manager tab until the 'freezing' is released (Control+Alt+Delete). The TM tab is still showing that nothing is unusual. The MB values are all in the low end of double-figures and, Firefox itself, in the low end of three MB. The only active extension is Ghostery but that is showing no signs of having operated for protection reasons or otherwise. Is Ghostery known to cause 'freezing'?

While I can continue with Firefox in Troubleshooting Mode, it would be more convenient to use normal mode. However with the above results and, having read the article regarding too much CPU etc, whether that is right in the light of the above.

Any further suggestions are welcome, otherwise many thanks for the above guidance. Kind regards, Tim Simpson

Sounds like there's something getting disabled in troubleshoot mode that's causing the problem. You'll probably have to do some testing to figure out whether it's Ghostery that's causing the lock-ups or not. The fix could be as simple as needing to update your graphics drivers.

Now that you know the problem doesn't happen in troubleshoot mode, the article linked below has some instructions to narrow down what particular setting or extension is not performing right.

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

I would try each suggestion listed in the article separately. That is, if you can't get the problem to happen with hardware acceleration disabled it's probably a driver issue. If it doesn't happen with Ghostery disabled it's probably an issue with that extension (and you'll want to contact the devs of the extension or switch to an alternative).

Does it still happen with hardware acceleration disabled?

Hi Support, Many thanks for your suggestion regarding disabling 'Hardware acceleration'. I did try this and it seemed to work. Additionally I did reinstall Firefox on both laptops and (after two weeks) it now seems satisfactory.

Many thanks for your guidance on this snag.

Kind regards, Tim Simpson