Firebug

[Snarky]

Which mayhem mission is the most effective for farming firebug?

[Piyerus]

Each mayhem mission has a chance of having an arson side mission, but the lower level ones have a limit on how many side missions will spawn, while higher level mayhems will have more of them. The last two mayhem missions (Founders Falls and Peregrine Island) will both have every possible side mission spawning every time, so those are probably the easiest bet if you want to cut down on random chance since you're guaranteed to get an arson job. Other than that its just a matter of what level you're able to farm at most efficiently.

[Generator]

Also, if memory serves, for both PI and FF, the arson key mob spawns pretty close to the front of the mission, if that matters to you.  (Sometimes it does to me, depending on my mood and what alt I'm playing.)

[Snarky]

Thank you all I made this way harder than it needed to be.  In the PI Bank mission there are knives of Artemis straight in from the door a block or so.  They have the arson key.  Run ten times.  done.