...not that the QuickTime folks at Apple are doing any better.
Both players can play .avi files. The standard .avi mime-type is video/x-msvideo. My server (and my Gallery2 implementation) send the proper mime type when embedding an .avi file.
Neither the Windows Media Player plugin for Windows Mozilla nor the Quicktime plugin for Windows Mozilla list video/x-msvideo as a supported mime-type. Even though Mozilla's documentation says they should.
Piss me off.
There's a workaround (involving finding the file in the pageinfo and downloading it) and a hack (involving changing the plugin registration file, but it didn't work for me). Not happy.