I keep hearing people say that Firebrand Imps are bad. They are not!!
An Imp has a 2 dice attack with a 1/3 burn chance. This is roughly equivalent (in damage) to a 3 dice attack. (An average burn token deals 3 damage over 2 rounds)
This means that the Imp has the same offensive power as a Fox, but instead of being Fast it has 1 extra HP and it is Fire Immune. Don't underestimate the tactical advantage of being able to Firestorm your own army, or lay down Fire Walls and walk straight through them. (This is an awesome way to counter a ranged attack heavy army)
Also, 6 health is a "sweet spot" where it is relatively difficult to kill in one hit. A 4-5 dice attack has a very significant chance of killing a 5 HP creature in one hit but will often leave a 6 HP creature with 1 HP.
Of course, the firebrand imp is 100% useless against anything fire resistant, but as a corollary it is really good against flowers and watchtowers and stuff.