You could do it with CSS tweaks and various add-ons before, but the latest nightly builds of the next major Firefox version suggest that users will be able to add and remove any toolbar buttons, from both the main forward/back/address bar and the tab toolbar.
That means you could, as Mozilla Links suggests, create a single omni-bar, albeit one that's hard to use after a half-dozen tabs. More likely, you can remove features you don't use, like the new tab button (for keyboard fiends) and the "List all tabs" tool on the edge. If there's one area where Firefox can win out in the browser game, it's customisation, so let's hope there's more to come. [Mozilla Links]