Just the fact that the address bar font is the same size as the rest of the UI gives me the impression that they didn't think it through. Downloading files has the same problem, it's relegated in a small button, while it's one of the main features.
The whole interface is one of the most unintuitive things I've ever used, considering how simple a browser can be in terms of functionality the user must access. I'll also never forget they they thought tabs on top didn't make sense (it's actually still an option you can change).
Chrome had search integrated in the address bar from version 1, every time I opened Firefox year after year and I saw that little separate input for searching I felt like using grandma's browser.
In terms of how pretty the UI is, Firefox 53—which just got released—looks better, but it took them years to get to where Chrome was back at version 1.
Syncing of bookmarks, settings, addons, passwords, tabs across devices etc. on Firefox is a lot less efficient.
From a technological point of view, it took them years to implement sandboxing, and they're still not done. If a tab misbehaves in Firefox it can take down the whole thing. Chrome had it right away.
I briefly tried switching to Firefox again a few months ago. Initially I tried fixing it via changing the interface via XUL, but the problems were too deep. I got very frustrated and went back to Chrome.