I'm confused about what you're even trying to do here. The overworld and dungeon palettes have exactly the same structure. Dungeon walls typically have 3 colors plus black, and including every wall set they only use 5 plus black, so such a huge ramp as you want makes no sense.
Are you even using the same Firebird set as I am? Is there something I'm not getting here?
Look, I'll go through the palette structure for you:
Overworld CSets:
0-transparent
1-white/light color
2- dirt [light]/browns color ramp/highlight color
3- dirt [dark]
4- non-tree brown color ramp
5- tree brown color ramp end [this is probably what's throwing you off]
6- non-tree brown color ramp end
7- item 1 ramp
8- item 1 ramp
9- item 1 ramp
A- grass [light]
B- grass [dark]
C- water/item 2 ramp
D- water/item 2 ramp
E- water/item 2 ramp
F- black
The only differences in between CSets tends to be the item 1/2 ramps.
Dungeon CSets:
0-transparent
1-white/light color
2-dungeon wall ramp
3-dungeon wall ramp [light floors as well]
4-dungeon floor ramp [some walls as well but very rarely]
5-dungeon wall/floor ramp
6-dungeon floor ramp [plus cave tiles]
7- item 1 ramp
8- item 1 ramp
9- item 1 ramp
A- grass [light]
B- grass [dark]
C- water/item 2 ramp
D- water/item 2 ramp
E- water/item 2 ramp
F- black
In dungeon palettes, CSet 3 is typically a flipped version of CSet 2, in that the 4th and 6th colors are given what the 5th color's hue was, and vice versa. It's used if you want to have a slightly different-looking part of dungeon while still keeping the same CSet as the overall one.
The most difficult bit to figure out about the palettes - and it really isn't hard to figure out, let's be real - is the dichotomy between the floor and wall color. Really, you could just go with a flat ramp for the whole thing and survive, but having the colors mixed like that allows for you to have different wall and floor colors in a single CSet.
The palette structure is slightly obtuse, sure, but it works perfectly fine. There's much more pressing issues with the tileset right now.