Anyway they've been releasing a lot of "beta' for Firefox now and has dragged the "releasing date" with them as well for sometime now. According to their release note the beta "12" version includes few improvements such as
*. Enhanced performance upgrade on flash player.
*. Better plug-in compatibility with hardware acceleration enabled.
*. They've fixed that annoying "feature" which used to display the URL whenever you point your mouse over a link on the location. Now with this update it is brought back to the default bottom window section.
*. Better security, stability support.
According to mozilla they hope that this would be the final beta release of the upcoming major Firefox 4 full version. And to be honest I haven't used this version because of the "experiences" that I had before so until they release the full version, think I'm gonna stick with the older version for now.
If you want to install it and try it out then go to this official firefox 4 beta downloading page.