Mozilla has completed the merge on Firefox 13 on to the Beta channel on April 26th. New stuff in Firefox 13 Beta includes:

• Redesigned Home Page: The Home Page now gives you easy access to Bookmarks, History, Settings, Add-ons, Downloads and Sync Preferences with one-click shortcuts.
• Redesigned New Tab page: When you open a new tab, you can easily get to your most recently and frequently visited sites.
• Tabs on Demand: Firefox loads tabs on demand when you restore a previous session, which makes it faster to restart Firefox windows with many tabs.
• SPDY Support: Firefox Beta now supports SPDY by default.

For more information or to manually download Firefox 13 Beta can be found here. Note: Existing Firefox Beta users should be able to update via Help > About Firefox.. Firefox 13 will be released around June 5tth and will also be the last version to support Windows 2000 and XP SP1 or older.