recently I upgraded to win8.1 and firefox 25, all flash cannot be played.
the solution is to edit
for 32-bit:
C:\Windows\SysWOW32\Macromed\Flash\mms.cfg
for 64-bit:
C:\Windows\SysWOW64\Macromed\Flash\mms.cfg
add a line "ProtectedMode=0".
This will disable the sandbox of flash so it is a little bit risky to do so.
