Now when you uninstall AIR 20, install AIR 19, and then try to launch an package, this attempt fails because the Adobe AIR Installer that is a 64-bit binary is unable to load the AIR Runtime that is now a 32-bit binary again.
That Adobe AIR Installer (version 20) keeps its precedence over the Adobe AIR Application Installer (version 19) that is present at The solution to this issue is to simply delete the directory that contains the Side Car package and Adobe AIR Installer so that this 64-bit binary longer exists.
Please ensure you run the installer as an admin and also make sure it is not running from a network drive since that cases all kinds of issues. It has always been mostly a browser plugin as far as I know.
/quotemsg] I want the Steam Flash Plugin which needs Flash.
Please see here for some basic trouble shooting of Flash under firefox.