Mozilla currently labels the Aurora releases in fact as "Aurora" browsers with a new logo and the mozilla-central versions as "Nightlies", also with a new logo. The transitions appear to be mainly testing the new rapid release cycle procedure and there isn't much to see for Firefox users yet. Firefox 4 will get the 4.0.1 (Macaw) security update in May before we will see any changes in the preview releases of Firefox 5. Among the key changes will apparently be the integration of the Home button, which will be built into Firefox as an app tab that will hide the Home desktop application for Firefox.
Mozilla said that Home will be offering a similar experience across all Firefox platforms including mobile and enable users to access all their information, including contacts, bookmarks, web apps and sync data via this tab.
June 21, 2011 UPDATE: Grab the download here.