The Mars was outfitted with selectable drop system in its later years.My understanding is that the -747, like the IL-76 used by Russia, basically have such a load wait that unless you have a Strategic Air Command Runway suitable for fully loaded B-52's...it's a no go.
Even the DC-10 and MD-87? passenger jets require major bases to reload off of.
Most of the larger tankers do not drop a "slug" effect like the Martin Mars did but more of a controlled jet on retardant, some of which may be air boosted to empty tanks, to create a stream of water that covers much more ground (i.e. more line controlled) and/or doesn't smash the timber underneath as bad making it safer for ground crews to follow up on.
Bigger is not always better...especially if you have limited infrastructure to support them.
Kinda interesting that it was considered to big. Now we are seeing 747 and such being used.