"In case you wonder, the reason for which Firefox doesn't merge url and search bar is to protect user's privacy. If you prefer a Chrome-style UI, there are a couple of add-ons that provide just that."
In my opinion, they need to think this through a bit more. Each input bar is crippled in some way (one won't autocomplete, the other won't go to addresses), and it's all driven by wanting to keep incomplete searches private (until Enter gets hit). Do users realize that the search bar in Firefox is not private, whereas the address bar is? I'm betting not. Do users expect them to be? Chrome assumes not.
In my opinion, if the privacy thing is so important, they'd still be better off having just one input with autocomplete off by default, and easily toggled to on. I doubt that users understand the current setup as "address bar is private, search bar is not".
I'd actually vote for one change, though: I'd prefer the URL field didn't default to falling back to querying a search engine. One of my pet peeves is mistyping a URL and having my input sent by my system to Google. (That's easy enough for me to change, though, and the Firefox developers seem to be concerned enough about privacy and security to leave the ability to change it intact.)
Essentially, I'm suggesting that they either merge the two fields or make them more distinct. At the moment, it doesn't seem accurate for one to be called "address" and the other to be called "search". The actual function is closer to "private" and "not private". Your suggestion is a good one for one of the directions they could go in, because it increases the distinctness of the fields.