The vast majority of the bug fixes are related to fine-tuning the core functionality and fixing stability problems. It's worth noting that Firefox Sync is now easier to set up, but you still need an e-mail address, a password, and a backup copy of a provided encryption key. 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 8 directly for Windows, Mac OS X, and Linux 32-bit/64-bit. The beta supports 57 languages.
Mozilla was originally planning on having Firefox 4 out by the end of the year, but it has had to delay the release till next year. Beta 8 was delayed till November 12, but clearly it didn't make the new deadline either. There's still a Beta 9 and a Beta 10 before we see the release candidate builds arrive, but all of those will hit in early 2011.