Wait so the ftb launcher uses the mojang launcher?
Wouldnt it be best to have the launcher launch the modpack using custom arguments?
Regards
loster31345ORlobster
Mojang specifically requested every custom launcher to use theirs to do the authentication and launching. I'm not sure if they actually took action against launchers that do not comply but even if they don't it probably is a good idea to honor that request.
As for why Mojang wants this: Probably to make sure user accounts are somewhat save as users don't have to rely on random third parties dealing correctly with remembering their user details as this is now done using Mojang's launcher.