Oh D'oh, I used MFTEnumEx in the WMF backend, but that function isn't supposed to be available on Windows Vista. I totally don't understand how we could event start up WMF on Vista given that we should be aborting WMF startup if MFTEnumEx isn't found: http://mxr.mozilla.org/mozilla-central/source/content/media/wmf/WMFUtils.cpp#331

MFTEnumEx doesn't work on Windows Vista, as advertised by the MSDN documentation. The reason we're not failing to startup on Vista is because we're not calling MFTEnumEx. DisableBlockedDecoders(void) in WMFUtils.cpp calls MFGetPluginControl(), and only calls MFTEnumEx if the call to MFGetPluginControls() succeeds. Unfortunately MFGetPluginControls() isn't available on Windows Vista, it's only available on Windows 7. So the call to MFGetPluginControls() fails and we don't take the "disable blocked decoders" branch, and we continue initializing. I'm not sure what, if anything we can do here.