Could you investigate this:

Multiprocess. Recent versions of Firefox are migrating more users to a "multiprocess" architecture where the user interface and the web content are in separate processes to improve stability. Firefox assesses whether your current configuration is compatible during the first run -- first run after installation or first run after a Refresh -- and if it is, enables it going forward for future runs. So this might explain problems that surface after a successful session and restart. To test:

Are you using Multiprocess (e10s)?

Multiprocess creates a second firefox.exe or plugin-container.exe process to isolate the web content from the browser UI. You can check whether you have this feature turned on as follows. Either:

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter/Return

In the first table on the page, check the row for "Multiprocess Windows" and see whether the number on the left side of the fraction is greater than zero. If so, you are using e10s.

If you are using e10s:

To help evaluate whether that feature is causing problems, you could turn it off as follows:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste autos and pause while the list is filtered

(3) Double-click the browser.tabs.remote.autostart.2 preference to switch the value from true to false

Note: the exact name of the preference may vary, but it will start with browser.tabs.remote.autostart

At your next Firefox startup, it should run in the traditional way. Any difference?

Thank you for your assistance. You nailed it. I understand the reason for multiprocess, and it's a step in the right direction for Firefox. Is there anything else for me to do on my end to help identify the malfunction?

It's rare. A couple month ago, Windows 10 users got an update that, combined with Norton security software and Malwarebytes Anti-Exploit, caused the identical symptoms and had the identical workaround. But I haven't heard of any other combination of circumstances/programs that cause it.

jscher2000 said

It's rare. A couple month ago, Windows 10 users got an update that, combined with Norton security software and Malwarebytes Anti-Exploit, caused the identical symptoms and had the identical workaround. But I haven't heard of any other combination of circumstances/programs that cause it.

Update: uninstalling Bitdefender anti virus allows Firefox to proceed as normal with multiprocess enabled. Seems to be an issue with their product. Will research further and see if there is a solution, as the software provides no indication that there is a problem.

Update: Some quick research online has identified that at least a few other Bitdefender users have encountered similar issues with the software in conjunction with Firefox. Re-installation of Bitdefender seems to temporarily solve the problem, but it doesn't last.