I have the latest flash player, 11.9.900.152, running on Windows 7 (64-bit), in the latest version of Firefox, 25.0.1, it will not shut down!
Look, I'm not a guru, I'm a retired technician, but still this is something disturbing.
I can go to a web site that uses the flash player and when I shut down the browser, the player is still active, sometimes playing the audio from the last application. If I launch a web site that should play a video, it may open the web site on the second try, but there are two instances of the audio from the video playing!
The web site: http://nos.nl/video/583035-krachtige-storm-trekt-over-nederland.html
I had been playing a slot machine on Facebook and after shutting down the browser, the background sounds from the slot machine game, some casino background noises, was playing an hour later!
This can be problematic, I can see how something could be send to my machine after a set time period and I would never notice it.
I have to manually do the three finger salute and kill the process. I will try to remove it and re-install it.