The only solution that makes sense to me is to use the rocket and send it under the level. I know that the blue "exhaust flame" counts as a fire source and can ignite the fireworks with the direct contact this would cause.

The problem for me is that it won't let me place the horizontal wall low enough to hold the rocket up... I can only get the first 3 fireworks on the left before the rocket falls down, and putting the horizontal wall at the next lowest height allowed makes the tunnel just a tiny bit too skinny for the rocket to fit into.

Am I using the wrong solution to this puzzle completely?

Or do I just have to be more pixel finicky with my placement to get the height right?