The card reads: "Any player may slot Fuel Tanks underneath a (Fire) card stack in their Command Line"
My assumption is it allows you to use Fuel Tanks to boost a 1 or 2 card (Fire) stack without having to change the top card, but is that all it does? Seems sort of weak.
I wouldn't call that weak at all. It doubles your chances of being able to boost an existing Flamespitter or Blaze without drastically reworking your Command Line.
Maybe not a first pick if you're playing 2-player and get to draft two cards a round anyways, but if you're only drafting one card, the additional flexibility in not disrupting your move-and-turn patterns is pretty awesome.