i rarely use the multiple playlist tab feature, but for people who do:

i think it might be very helpful to show an asterisk (*) in the tab that's currently playing music. or, maybe, some other type of symbol/representation.

wouldn't this be a good thing to include in fb?

Foobar2000 always plays from the visible playlist. If you switch playlists while a file is playing, it will start playing from the new playlist as soon as the current file ends.