Can passengers in an open topped vehicle (particularly a Dark Eldar Raider) fire on targets?
Yes, as long as the vehicle moved no more than 6" that turn.
Are they limited to the same targets the vehicle targeted?
No.
How many of the passengers can fire?
All of them.
How slow/fast can the vehicle move and the passengers still fire?
As mentioned above: up to 6", no matter whether they are fast or not.
Have look at the open topped section and the "fire points" box in the transport section.