The vast majority of the bug fixes are related to fine-tuning the core functionality and fixing stability problems. It's worth noting, however, that the bookmarks and history code has been overhauled, enabling faster bookmarking and startup performance. At the same time, per-compartment garbage collection has now enabled, reducing work done during complex animations. For more information, check out the release notes.
If you're already running a previous beta of Firefox 4 you should see an automatic update soon. Alternatively, you can download Beta 9 directly for Windows, Mac OS X, and Linux 32-bit/64-bit. The beta supports 76 languages.
Mozilla was originally planning on having Firefox 4 out by the end of last year, but it has had to delay the release till next month. There's possibly still a Beta 10 before we see the release candidate and final builds arrive.