Lets say for some reason you play MW2 with a nonsteam patch ( or for some reason xfire wont recognize your MP ) then use this info on how to active Xfire ingame,
First go here
C:/Program Files/Xfire/ then open the file xfire_games
then go to the very bottom,
paste this over the last entry which should be a 
LongName=Call of Duty: Modern Warfare 2 Multiplayer
Launch=%UA_LAUNCHER_EXE_PATH% %UA_LAUNCHER_EXTRA_ARGS% %UA_LAUNCHER_NETWORK_ARGS%
Then go to xfire->settings->games->not installed , navigate to MW2 MP, hit browse, find where its installed, hit ok , hit apply, and it will work ingame now.
Credits to vidgam904 for making this guide. I just posted it here thought i might help.
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