I think the firefox logo uses very bright and saturated colours, so you should try to use them as well. In my edit you can see i made the shades very saturated and less dark than yours. I also noticed that your clusters weren't looking very nice, so I tried to improve those, its kind of difficult to explain what makes a good cluster, but basically you wan't them to be as blocky as possible. You also want them if they have diagonal lines, to have those lines be straight so only steps down of 1 or 2.
Edit: 
Thank you so much

I knew that my color choices were slightly off, but I wasn't quite sure how. The advice on pixel clusters is also something I'm going to try and work on.
Honestly you picked a really hard thing to pixel. The firefox logo is almost all gradients, and I really don't know where to begin with the tail. Sorry I couldn't be of more help. 
No worries, I appreciate the attempt.
In terms of the difficulty - kind of yes and no. I think you're right in that if I were to try and recreate all the gradients on the official logo I would be in for a hard time. But the thing about pixel art is that it's about overcoming limitations - because the style itself has limits, you approximate and use certain restrictions to your advantage and come up with something cool in it's own right. Well... that's how I've always thought of it.
