Try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

Wesley,

When I restarted Firefox in safe mode, the problem went away.

Your response got me to thinking that the issue may have been a theme that I recently installed.

So, I restarted Firefox to swap themes. And, wouldn't you know it, once Firefox came back up, the issue had gone away.

Obviously errors do not magically "go away". Something in my execution environment had changed, and that change was the cause of the pop-up. The restarting of Firefox reset the environment (thus causing a reset of the offending component).

I suspect that in my attempts to try different themes, one of them must have left some residual data in storage, or, perhaps even overlaid some storage - with the result being the reported behavior.

But, your suggestion to restart Firefox in safe mode seems to have addressed the issue.

Thank you for your help, and for your quick and detailed response.

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.