This issue can be caused by an extension that isn't working properly.

You can check in View > Toolbars to see which toolbars are available and visible.
If you see a toolbar in that Toolbars sub menu possibly added by an extension that can't be displayed then see if you can find an update of that extension or disable (uninstall) that extension.

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

Thanks for your help. I checked the toolbars menu as suggested. I see nothing there that fits your description. The options are: Menu Bar, Navigation Bar, Bookmarks Bar, Add-on Bar (Ctrl+/), then Tabs on Top, plus Customize. All of these are standard (I believe) and not something I added. Besides, they all appear/disappear when selected/deselected.

As I mentioned before, this message appears in the place where I would normally get messages that say "tried to redirect", or "blocked a pop-up", etc. And I'm not getting those messages now, which makes me think something is corrupted. While I was waiting for a response, I uninstalled Firefox, rebooted and then reinstalled, and finally, updated via the Help menu, just to make sure I was 'up to date'. I did NOT remove personal settings when uninstalling/reinstalling. I still get the 'please re-install the toolbar' message.

With that in mind, do you have any other suggestions? And thanks again for the help.