This bug systematically occurs when exiting from Firefox. Acroread appears as process only, i.e., there is no acroread window, and uses a lot of CPU resources, that is over 40%. I had to kill it by force for releasing the use of CPU.
There is no firefox addon related to acroread in my Firefox config. This bug appeared just after the upgrade from 11.4 to 12.1
So far, the only solution I found is to remove acroread, and use alternative pdf readers.
Do you have any clue about the cause of this bug?