I switched back to Firefox and upgraded to beta, which comes with a new theme called Australis. I can sum up my beef with the new theme in a single image.
See the browser image on the far right? That’s Australis, taking up an unnecessary amount of vertical space, with fewer options (as near as I can tell) to limit or reduce it. I made this comparison using screenshots of the four browsers I had at hand, in full screen (where the comparison seemed easiest and most apt).
I didn’t capture it above, but the curve of the tabs also looks awkward and unprofessional. The curves introduce extra, unnecessary gapped space around the sides of the tab, as well, though I can see a case for that vis-à-vis Fitts’s law1. While I’m pretty unhappy with it on the whole, the design is a step in the right direction, and I’m glad Mozilla is trying something out. It’s just not my sort of thing.
Luckily, I was able to tweak things (especially the pernicious top gap introduced above the tabs) with an extension called the Classic Theme Restorer, which comes with enough options to suit me okay. It let me remove the top gap, square the tabs, re-enable small icons, and gave me back a few more features I was missing.