Do a clean installation of Flash for Firefox.

First, grab the Adobe uninstaller from here: http://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html

Then download the latest Flash Plugin version from here: Select the Windows > Plugin-based browsers - the EXE version. And you need the 32-bit version for ESR, which is 32-bit application. https://www.adobe.com/products/flashplayer/distribution3.html

Run the Flash uninstaller program, then reboot your PC. Then install the Plugin version of Flash - make sure Firefox is closed before starting the installer. Then reboot your PC. Hopefully that will solve your problem with the failed Flash update

As far as Flash working in Opera and not in Firefox - they use separate versions of Flash. Firefox uses the NPAPI Plugin version, while Opera probably uses the Pepper version (with Chrome), and IE uses yet a 3rd version - ActiveX. No direct comparison is possible - Adobe has different problems with each of the three versions of Flash.

Thanks.

More or less did that, but without the official uninstaller, used the Windows uninstaller.

I'll download the various versions and try again tonight, have to get out now. I'll post to report the results.

Thanks again.

Tried the solution posed by the-edmeister, but alas got the same messages, only now they stayed visible for longer. It appears that Kaspersky was being very concerned about my Firefoxes health. After closing all browsers, temporarily stopping Kaspersky, reinstalling Flash with the downloaded installer and immediately restarting the computer (so Kasperky works again) Adobe Flash now works as should. Alas some sites I visit heavily lean on flash, so I can't ditch it yet.

Thanks again for the help.