I realize that I'm in the minority as a power user but this would/will seal the deal on me leaving Firefox behind.
Also, eliminating the address bar seems to be a trend among the major browsers, if anything I would expect Firefox to be easier to revert the behavior (see how easy it is to switch back from tabs-on-top, for example).
Basically, here's my beef: I own both an iPhone and a computer with a 30" Dell monitor. They are not even remotely alike. Design decisions aimed at making the browser on my PC look and work more like the one on my iPhone are wrongheaded, misguided, and arguably batshit insane, from the perspective of someone who has actually seen, and in fact regularly uses, both platforms.
Vertical space, for one thing, is not at a premium on my PC. It should not be treated as a resource to be conserved at all costs.
To do otherwise would be similar to proclaiming that all movies must be made for children, because children will want to watch them.
As long as there's an easy way to invoke the url bar, I'll be happy.
Although one thing I'd like, which I've seen no one implement yet is to have links urls as tooltips on mouseover. Perhaps below another tooltip if it existed. Shorter distance to move my eyes.