I Need Feedback

It's a good start. Could definitely use some refinement. Here's a couple things that you need to look at:

1. The foxes hit box seems a little off. In places where it doesn't look like I should be touching the spikes I am and where I should be, I'm not. Also, in places where I try to jump from a lower platform to a higher one, I seem to get caught in a weird way.
2. The ground tiles don't line up on the side so there is a jarring break where they meet. Take a look at the image below. The top is your tile and the bottom is a modified version lining up the edge.
3. The difference in resolution between all the sprites is VERY distracting. I would suggest picking a single resolution and sticking with it.
4. The spikes aren't lined up with the tiles causing weird overlaps between them, the ground and other spikes.
5. The Red and Blue buttons on I believe the 3rd level don't seem to reset when restarting the level.
6. The level with the Red and Blue buttons, I can drop straight off the first platform and skip the rest of the level and the Boss level. Not sure if this in intended but it doesn't seem right.
7. The restart button on the Boss level places the fox back at the beginning and the boss back in it's start position but the boss doesn't actually restart. It just stands there unmoving.
8. The second level after the boss, if you fall off the platform, you fall forever forcing you to restart the entire game. This is also possible in part of the level right after the boss. Just drop to the platform below the gray bar and you can fall forever.
9. The restart button at the end of the game doesn't appear to do anything.

As I said starting out, it is a good start. You've got a bit to go before it can be considered a finished product.

Edit: I just noticed, the Red bars behind the boss (Lasers?) also don't stop the player from passing, just like the wall with the buttons.

Edit 2: It looks like the fall for eternity issue can actually happen on any level. Just walk off the level to either side. If you want to test this, just start the game and walk left off the screen instead of walking right.