Haaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa!!! Okay, actually,
skip made me think of it!
What about
Gloin???
I'm a firestarter, angry firestarter - he and his brother were the best in making fire, but also struggled during it
At gates of doom I saw too soon,
The slaughter and disaster. - not sure, could be anything, the gates of the Goblin-town, or perhaps most likely Erebor, or even in Moria if he was there? I don't have the patience to check now
White from blue, no minstrel, true - well, from Blue Mountains (that was what made me think) and, the Dwarves were all playing some instruments, though I think his instrument is not mentioned. He had white beard. And white cloak.
Torchbearer and light caster.- I guess this just refers to the fire again, or maybe something else... aaah, was he the one to accompany Bilbo inside the Mountain with the light? Might be!
Ninth in line, renown is mine,- he came as the ninth of all the Dwarves! You know, Balin, Dwalin... etc
And of course he is the only one who appears in FotR.
But student beats the master.- Maybe that's referrence to his son, Gimli, who ultimately "beat" him in significance.