More detailed steps if you run into permission issues editing the mms.cfg file:
(1) In a My Computer or Windows Explorer window, open this folder:
If that folder does not exist, then you are using 32-bit Windows, and you can open the following folder instead:
(Note: This folder exists on both 32-bit and 64-bit Windows, but on 64-bit Windows Firefox uses the Flash player in the SysWOW64 folder instead.)
(2) Check for a file named mms.cfg:
(A) If mms.cfg exists, drag it to your Documents folder where you can edit it without being bothered about administrator privileges
(B) If mms.cfg does not exist, open your Documents folder, right-click > New > Text File and name the new file "mms.cfg" (putting quotation marks around the file name help prevent Windows from tacking .txt onto the end)
(3) Open mms.cfg from Documents into a text editor such as Notepad. Add this on its own line (I put it last):
Save the file and close Notepad.
(4) Hold down the Ctrl key and drag the mms.cfg file back to the Flash folder to make a copy there, keeping the original in Documents
This change should take effect the next time you exit and restart Firefox.