Overall I like what I see with the new gradebook. Only thing that bothers me a bit is the text in the sub menus. It appears fuzzy and not as sharp as the other text in the gradebook.
I don't see that much difference between the two browsers but it is noticeable in your screenshots. I am worried about the contrast not being enough especially for people with failing eyesight like me.
As a card-carrying (and not very proud) member of the "over-50 club" and one who wears bifocals, I usually get pretty finicky when it comes to the text I'm seeing on the screen. I see what @chofer is seeing, as well, with slightly darker text in Firefox, yet I work mostly on Chrome and was not really bothered with what I would call fuzzy text when I was looking at beta the past few days. Here's my screen captures, for what it's worth...to add to the mix:
Is the Gradebook different from other areas of Canvas? Sometimes the resolution display of the screen where you are viewing Canvas can make a difference in how you view the images.
The New Gradebook also isn't completely accessible, at least to screen readers, though I don't know if contrast has completely been finalized yet (this is one reason New Gradebook is still considered to be beta, aside from the fact full functionality is still being developed). We may all just have to wait and see how our accessibility team continues to refine the feature.