According to the Mekanism Wiki there are a couple of modes for the flamethrower.
I am mostly interested in "Combat mode" to avoid turning every piece of sand into glass around me :D
You can switch between three fire modes using Shift + Item Mode (default M).
I tried this, and nothing happened, the flamethrower continued to destroy/change blocks it hit. I made sure none of the required keys were bound double.
I would like to use this weapon against mobs, but right now it collects dust in a chest.
Ive tried all the following keys (with and without also holding shift)
F (saw on github)
M (item mode switch)
H (feet mode switch)
G (armor mode switch)
U (mekanism voice)