You're right—what Mozilla should really, ideally do, is revert back to the old behavior, with no option to re-enable the new one. :)
Just about any other design would be fine, actually; I'm not married to the old one. No other browser has a URL bar I find particularly offensive.
I would bet a very large sum of money that if you sat every person in the world down in front of two computer screens, and let them play with both URL bar designs side-by-side, a large majority would say they prefer the old design. Not everyone, to be sure—humanity will never agree on what is most aesthetically pleasing—but most. That is why the modern field of graphic design focuses on grids systems and consistent spacing and margins—because most people find them appealing. (Not even to mention the other issues around covering the bookmarks and tab bars.)
If I seem a bit overly worked up about this, it's because working in the graphic design industry has made me very sensitive to poorly-aligned visuals, in the same way type designers become sensitive to bad kerning. And I live in my web browser.
I'll eventually adapt, as most people will. That does not mean this is a good change.