FF changes the UI very frequently. Even the last update from 42 to 43 had changes. I love FF and its my only browser but this is one of the most irritating things they do. Maintaining a standard UI is a must to retaining users (and keeping them upgrading).
I think you are underestimating the impact of the subtle nature of the differences. For me, it really kind of annoys me that FF seems to have a looser UI than Chrome. It's those couple of pixels of padding between address bar and tabs, it's the wider/longer tab due to padding, etc..
And now that I think about it, it's even things like the tab behavior where Chrome condenses tabs and FF just pushes them out of view an adds the disgusting scroll UI. Maybe that's a personal preference thing, but I would very much prefer having a dense overview rather than having what is essentially a nested scrolling.
I'm assuming there's an extension to merge the URL and search bars to get Chrome-like behaviour?
Yes you can enable search suggestions in the URL bar right in the Firefox preferences and hide the separate search box. I keep them separate because I don't care to have everything I type in the URL bar be sent to Google by default.
Exactly. The divide between address bar and search bar is a much under-appreciated privacy feature.
The new FF UI has been out and stable for nearly two years now.