You made me count, because I could have sworn it was thinner than the top bar, but it's a bit more complicated than that. On a 4K display the single-icon vertical tabs on Firefox are 75 pixels wide. The horizontal tabs bar is a sliver narrower, at 65 pixels tall. Of course that stacks on top of the address bar, which itself is 60 pixels tall, so you end up with 125 pixels of top bar.

I don't know if I could notice the 10 px difference between the two, given that they're in different orientations and 10 pixels is 0.5% of the horizontal pixel count and 0.3% of the vertical, but human perception is weird. Like I said, I keep the bar much wider to read the titles and just... hide it when I'm not tabbing, so it's not an issue at all for me. Although I'll say that even with the wide sidebar deployed you get a pretty comfy square-ish space to work with that turns a 16:9 display to 16:10 in a satisfying way. And on ultrawide 21:9 it's a no-brainer, just like having a side-aligned taskbar (hear that, Windows 11?).

I should add that none of that changes that Firefox is... quite ugly in general. Zen is definitely sleeker at a glance, regardless of your setup.