There are imho unnecessary padding and huge font sizes. For example: http://imgur.com/Cvzky1K.png
1. Nice contrast, good padding 2. Why not the same padding as 1.? 3. To much vertical padding. Contrast: are those deactivated? 4. To much vertical padding. 5. Why is this font so huge?
Or why is this sidebar font so huge: http://imgur.com/DZO4l4D.png
But I have to admit, the situation got better not worse in the last versions.
But Chrome imho also has the smarter solution in the network panel: workflow over more panes. You click a request and you see the details in the main area replacing the waterfall, not adding another pane fighting for space.
[edit] All a little bit smaller on the left: http://imgur.com/DooYnFY.png
Or is it the tab icons? Chrome recently disabled those by default, and I'd agree that they do seem like clutter.
Edit: Misread the comment you were replying to. I thought you were comparing the number of icons and tabs, but you're just talking about the size of the elements. Firefox is slightly larger, yeah.