Your System Details shows you have the current flash.

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

http://helpx.adobe.com/shockwave/kb/troubleshoot-shockwave-player-installation-windows.html Troubleshoot Shockwave Player installation | Windows http://helpx.adobe.com/shockwave/kb/troubleshoot-shockwave-player-installation-mac.html Troubleshoot Shockwave Player installation | Mac OS

https://support.mozilla.org/en-US/kb/adobe-flash-plugin-has-crashed

https://support.mozilla.org/en-US/kb/troubleshoot-issues-with-plugins-fix-problems

Try to re-download the Flash DMG disk image file.

You can download the latest Flash player versions for Firefox on this page:

• https://get.adobe.com/flashplayer/otherversions/

I re-downloaded the dmg for Firefox and went to downloads, tried to open and got a warning message again that it cannot open the disk image - error 4960

A Google search suggests to re-download the DMG file. If that still gives a corrupted file then you may have security software that is corrupting the file (the file might be signed). If you have security software then try to disable it temporarily to see if that works.

See also:

• https://wiki.mcneel.com/rhino/mac/disk/image/could/not/be/opened/rhinoinstall
• https://www.google.com/search?q=mac+error+4960+disk+image