I was wondering. If you want to throw a grenade for example a fire bomb, you successfully managed to throw it and it landed in front of an enemy...does the grenade blow up instantly or is there a time for it to explode for a example a certain number of rounds? And if so, does it work for all the grenades or certain ones, like the frag grenade?

In DH, grenades are "timed" to go off directly after a successful BS test, or after you've determined where the grenade really ended up after a failed roll. But nothing prevents you from having grenades that go off after a round or so (in reality grenades usually have a time delay of about 2-5 seconds, depending on the type), but that would be something you'd have to talk to your GM about.