Acceptable creatures with firebreathing must be able to spend one mana of any color to get +1/+0 until end of turn. Frozen Shade doesn't count because it gets +1/+1, not +1/+0. As long as it gets +1/+0 for the expenditure of mana, nothing else matters.
I'm also adding 'it must then be able to attack' to the list of qualifications. For example, Krovikan Elementalist can pump himself...but then he can't attack, so he doesn't count.
Finally, gold creatures will not be acceptable. Sorry, but even if Ranger En-Vec is a green creature with first strike, it's also a WHITE creature with first strike, and those are way too common.
[Edited by Istanbul on September 25th, 2000 at 11:58 PM]