Type about:plugins<enter> in the address bar. Now search for the plugin.

Example; Adobe Acrobat File: nppdf32.dll,nppdf32.dll Path: C:\Program Files (x86)\Adobe\Reader 11.0\Reader\browser\nppdf32.dll,C:\Program Files (x86)\Adobe\Reader 11.0\Reader\AIR\nppdf32.dll Version: 11.0.9.29 State: Enabled Adobe PDF Plug-In For Firefox and Netscape 11.0.9

This will show you the path to that program. Copy the Path Only. Also, note the name of the file, and remember it. Now Close Firefox. Open your file browser and paste the path in the address bar. Find the file, and delete it. Now you can restart Firefox. Warning ! Remember to delete the right program.

To be safe, remove all of them. Then install the current.

I followed the instructions above and then tried running Adobe Flash Player Installer and it still fails at about 55% complete.

Start your Computer in safe mode with networking. Then try the installer. Is the problem still there?

Starting The Computer In Safe Mode;
Free Online Encyclopedia

Adobe Support https://forums.adobe.com/community/flashplayer

The above didn't work but I did follow the link to the Adobe Forums and found this thread:

https://forums.adobe.com/message/8360535#8360535

Where there was a suggestion to use the Standalone installer (what ever that is). Anyway that worked.

Thanks everyone who commented. Your help was much appreciated.

That was very good work. Well Done.