The devs should contact Xfire and get Tremulous supported. I made custom code for it awhile ago but unless the devs actually contact xfire at newgames@xfire.com it will never be officially supported.
[xxxx]
LongName=Tremulous
ShortName=trem
LauncherDirKey=HKEY_LOCAL_MACHINE\SOFTWARE\Tremulous\InstallDir
LauncherExe=tremulous.exe
LauncherRconArgs=+rconpassword "%UA_GAME_RCON_PASSWORD%"
LauncherPasswordArgs=+password "%UA_GAME_HOST_PASSWORD%"
LauncherNetworkArgs=%UA_LAUNCHER_PASSWORD_ARGS% %UA_LAUNCHER_RCON_ARGS% +connect %UA_GAME_HOST_NAME%:%UA_GAME_HOST_PORT%
Launch=%UA_LAUNCHER_EXE_PATH% %UA_LAUNCHER_EXTRA_ARGS% %UA_LAUNCHER_NETWORK_ARGS%
ServerStatusType=Q3A
InGameRenderer=OGL