Sonic Firsts is just a collection of reprints that features four first appearances:

"Don't Cry for Me Mobius"
(Reprinted from Sonic the Hedgehog #0, limited series.)
First appearance of: Sonic, Tails, Princess Sally, Rotor (called "Boomer" back then), Antione, and Robotnik.
StH #0 was also the *only* appearance of Sally with blonde hair, but you can't tell from this reprint!
They undermined Barry Grossman's work by re-coloring the whole re-print!  And I thought they only did that to Spawn figures!  *sigh*  here's an example:

 The image on the left is from the original print of StH #0.
The image on the right is from Sonic Firsts.
If you didn't know about this & picked up StH #3 from the regular series, you'd be sratching your head over the 1-page
"Princess Dye" skit.

"Rabbot Deployment"
(Reprinted from Sonic the Hedgehog #3, regular series)
First appearance of: Bunnie Rabbot
 This story was actually the back-up story in StH #3 (regular series), which followed the main story, "The Bomb Bugs Me". (Click here for page 1 of that story.)  Micheal Gallagher & company try to explain how Bunnie came to be part robot.  This also set the stage for those "hare salon" gags.  It also contained a few puns that made me laugh out loud.
Here's my favorite:

Once again, the reprint was re-colored, as evident from these scans of the last panel.

The image on top is from StH #3.  The image below it is from Sonic Firsts.  See the differences?
Actually, Karl Bollers *improved* the colors on some panels (in the reprint).  For example, in the original print,
"team America" (Linda & Daryl America, that is)  didn't color in the characters' eyes in some panels.  Still, I don't
like the way Bunnie looks with lime-green eyes (as Karl keeps coloring them) *YUK!*  What gives, Karl??

"The Lizard of Odd"
(Reprinted from Sonic the Hedgehog #4)
First appearance of: Super Sonic, Universalamander
Robotnik creates a huge, monster robot out of a little salamander.  However, the Universalamander had no interest in following evil Ivo's orders and, instead, set out to conquer Mobius for himself.
This was the first time our blue boy went golden (after being chomped by Universalamander a few times first, just to show us how badly Sonic needed a juice-boost in order to take care of this bad 'bot.)
Unfortunately, I do not have a copy of the original, but I *know* at least Sally was re-colored from a pink-furred brunette to the brown-furred red-head we see today.  I know this because Sally was colored with "pink" fur from StH #2 (limited series) through StH #15.

"The Island Hedgehog"
(Reprinted from Sonic the Hedgehog #13)
First appearance of: Knuckles the Echidna
Well, I couldn't tell if there were any "color changes" between the two prints.  Sonic and Tails discover the Floating Island in this story and investigate.  Unfortunately, Robotnik beat them there and convinced Knuckles, the island's Guardian, that Sonic has come to steal the island's Chaos Emerald (which is the thing that makes the whole island float in mid-air).  While Knuckles dealt with Sonic and Tails, Robotnik stole the Chaos Emerald.  When Knuckles returned to the Chaos chamber and discovered that the Emerald was gone, he realized that Robotnik had played him like a violin.  Knuckles and Sonic called a truce, recovered the Emerald, and kicked Robotnik off the island.