Mozilla hired Radiant Core, a company from Toronto, to design the new theme. IT Business says that the "criteria for the [theme] proposals included that they respect native OS look and feel (Firefox runs on Windows, Macintosh and Linux platforms), that the themes appear modern and contemporary with current Web and client apps and appear consistent across platforms."
So now we have translucent buttons, an improved search box and new tab strip that shows the active tab prominently. There's also an arrow next to the last tab that hides a list of all the open tabs.
You can download Firefox 2.0 Beta 2 for:
Windows (5.6 MB) | Mac (18 MB) | Linux (9.2 MB) | Other versions
In order to be able to use the incompatible extensions, follow the tips from here.