Update to 58.0.2 from https://firefox.com

Then do a Refresh:

The Refresh feature (called "Reset" in older Firefox versions) can fix many issues by restoring Firefox to its factory default state while saving your bookmarks, history, passwords, cookies, and other essential information.

Note: When you use this feature, you will lose any extensions, toolbar customizations, and some preferences. See the Refresh Firefox - reset add-ons and settings article for more information.

To Refresh Firefox:

1. Open the Troubleshooting Information page using one of these methods:
   • Click the menu button , click help and select Troubleshooting Information. A new tab containing your troubleshooting information should open.
   • If you're unable to access the Help menu, type about:support in your address bar to bring up the Troubleshooting Information page.
2. At the top right corner of the page, you should see a button that says "Refresh Firefox" ("Reset Firefox" in older Firefox versions). Click on it.
3. Firefox will close. After the refresh process is completed, Firefox will show a window with the information that is imported.
4. Click Finish and Firefox will reopen.

Did this fix the problem? Please report back to us!

Thank you.

Refresh didn't help. I tried completely uninstalling from the Control Panel - I had both version 53 and 58 installed so I thought maybe a clean install would help.

Reinstalled version 58 and it's slightly better, but still fluctuating between 80 and 90 degrees CPU temp in the quick test I did. I'll try again later and see what it does in a longer test, but I don't think that's fixed the issue.

In case you are running any Trend Micro security (related) product as apparently shown in your crash reports (see tmmon64.dll), I would first try disabling any of its components "interfering" with Firefox.

Have you tried creating an exception inside that product for both the Firefox program files and your Firefox profile folder?

You could also do some performance monitoring using the Resource Monitor (e.g. using Task Manager > Performance) to see what process is actually causing the CPU load to be probably higher than normal, resulting in higher temperatures. Restarting Windows in Safe Mode with networking enabled is a reliable way of temporarily disabling any antivirus product, rather than just disabling it.

Disabling Trend Micro isn't an option unfortunately, as it is a corporate laptop. I've looked at Task Manager, and the usual behaviour when Firefox launches is that Trend Micro will take up some CPU, but generally less than 10% during Firefox startup and then gradually subside. 'System' is the process that takes up most of the CPU - often around 50%, with Firefox taking around 30-35% at times.

- Can you disable the Trend Micro add-on in Firefox if available? - Can you still add an exception for Firefox inside the program settings? - Is your Trend Micro product fully up to date?

Also see here and here.

I’m not entirely sure, but perhaps any load caused by Trend Micro (such as scanning the Firefox profile folder) may appear as system load in Performance Monitor. In case of misunderstanding: the Resource Monitor - not the Task Manager only/itself - is your friend for monitoring active tasks such as memory, disk etc..