Currently there are more than 50 blockers that still need fixing and more than 300 bug fixes which need to be tested before the finished code is released. Although right now there is no plan of releasing a beta 12 of Firefox 4, if the planned fixes land too late, there will be a beta 11 release next Tuesday and a beta 12 when all blockers are at zero.
The release notes for Firefox 4 Beta 10 are pretty light on details. Mozilla mentions stability improvements for Flash under Mac OS X, lowered memory usage, as well as support for a graphics driver blacklist. If you're already running a previous beta of Firefox 4 you should see an automatic update soon. Alternatively, you can download Beta 10 directly for Windows, Mac OS X, and Linux 32-bit/64-bit operating systems.