It's a few days late, but Mozilla announced the availability of the second Firefox 4 beta today.
Ars Technica's Ryan Paul said, "One of the major user interface additions in beta 2 is support for 'application' tabs. When you convert a regular tab into an app tab by selecting the relevant option from the right-click context menu, the tab will shrink down to just the icon and move to the far left-hand side of the tab bar. When the implementation is complete, the app tabs will eventually persist across sessions."
Firefox 4 is also improving rendering by handling emerging Web standards. CSS3 transitions and transformations are supported in the beta.