Even though the Adobe Flash plugin (18.104.22.168-0precise1) is working fine in the Firefox (15.0.1) window embbeding it, it becomes visible on other graphic application windows covering it. That is, they become "transparent" to the flash image. This is on Linux Mint 13 Maya 32-bit Mate, and I have observed this problem since I have installed this OS in July (i.e. it remained along the versions in this period). No such problem occurs in Ubuntu 12.04 32-bit, installed in another partition of the same machine. If relevant, the NVidia driver is 295.40.
Is it a bug? (I have searched this site but found no similar report so far).