Could you test in Firefox's Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the option/alt key when starting Firefox. (On Windows, hold down the Shift key.)

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

If so: a likely culprit would be an extension. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Command+Shift+a (Windows: Ctrl+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. Then cast a critical eye over the list on the right side. Any extensions that Firefox installs on its own are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious? If in doubt, disable.

Sometimes a link will appear above a disabled extension to restart Firefox before the change takes effect. You can complete your work on the tab and click one of the links as the last step.

Any improvement?

See also:

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

I tried all of this, but subsequently found that the problem was a piece of malware called "Advanced Mac Cleaner" downloaded along with the Firefox update. I do not know how it got attached to the Firefox update. but I have removed all of the malware files as directed by an Apple notice. Thanks David Beck

Where/how did you get that update?

I am glad to hear that your problem has been resolved. If you haven't already, please select the answer that solves the problem. This will help other users with similar problems find the solution more easily.

Thank you for contacting Mozilla Support.

In one of the many pop-ups that I received, I found the name of "Advanced Mac Cleaner exhorting me to subscribe. Looking up that name, I found the following link:

https://discussions.apple.com/thread/7142076?start=0

After following the directions to remove this malware, I have been relieved of the problem.

DBeck said

I tried all of this, but subsequently found that the problem was a piece of malware called "Advanced Mac Cleaner" downloaded along with the Firefox update. I do not know how it got attached to the Firefox update. but I have removed all of the malware files as directed by an Apple notice. Thanks David Beck

No version of Firefox for Windows, Linux or Mac OSX from www.mozilla.org/firefox/all/ or internal updates comes with any additional software or malware.