Why does Yahoo Messenger close when I sign in to my Gmail?

ok, I do have 2 Yahoo Messenger accounts, that work fine when I sign out of one and into the other. However, if I turn on Yahoo Messenger, then I go to sign in for my Gmail, Yahoo Messenger closes. I am new to Gmail, maybe there is a chat feature I need to turn off? I get to have other chat programs open at the same time, why not Gmail & Yahoo messenger? Is this a FireFox issue? That is the browser I am using. Thanks for the help!