I like the Firebombs with Flash Fire a lot, actually. By targeting the ground with the initial throw you can hit enemies off-screen and around corners with the bounces without exposing yourself directly to danger. You can actually out-range Mortar enemies with that trick.
But they just don't do enough damage. 85% is just over half the single-target damage of Splinters, and if you're using them for indirect fire you're wasting the initial full-power hit so that the first attack is only doing 60%. Useful as I find them I can't justify ever using them instead of Splinters, especially for bosses. I'm using them plenty on Inferno but they're always supplementing Splinters, never replacing them.
I'm seriously considering dropping them for another survival skill, though. I already replaced Soul Harvest with Fetish Ambush and as things keep ramping up I'm probably going to start seeing doubling up on attack skills less and less favorably.
I should also really drop Zombie Dogs, but I've taken them this far already and I'm too stubborn to give up on them now.