Ive been really happy with this build and Id recommend it for both multiplayer and singleplayer. My Firefly is level 40 atm and finds most content very easy (except fire immune badmmuthas
Basically it's an elemental siren with incendiary weapons and a class mod that give +incendiary and bonuses to skills like; spark, diva and phoenix. Skill progression is Spark -> quicksilver -> intuition ->phoenix -> diva -> inner glow -> girl power -> mind games.
I played around with a lot of weapons but settled into 90% sniper/ 10 % rocket launcher. Both have 4x incendiary. Due to the skills and mods, incendiary nearly always goes off and causes a large explosion of fire. This happens even if you hit the ground and, because the majority of damage comes from fire procs (weapon and pheonix skill), you can use a sniper rifle in close fighting and it works well. Hitting them is a bonus
. This build tends to kill badass/badmuthas of an equal level with 3-6 sniper shots (non crit).
In multiplayer, the XP bonus from intuition is a nice and you can sit out of combat and do insane amounts of fire damage with the sniper rifle. In really big packs and indoors, a 4x incend rocket launcher that fires 3 shots per rocket will lay waste. Phoenix really helps ammo problems in multiplayer.
For fire immune bosses a nice shock SMG with a +shock or a +SMG damage mod will get you by. Its also handy to bring out for any mobs with big shields.