Mozilla has been released the tenth beta of Firefox 4 with fixing many issue that you can be access here. This fixed issue include improvements in memory usage, compatibility and stability improvements when using Adobe Flash on Mac OS X and support for a graphics driver blacklist to improve stability.
Firefox 4 Beta has new feature like grouping tabs, Firefox Sync which you need additional add-ons on Firefox 3.6, and new looks for manage add-ons and theme. In Firefox 4 beta, manage add-ons will not show in popup window but in new tab like on Opera 10.
If you have Firefox 4 Beta installed on your computer, you can update from Software Update on Firefox 4 Beta (Help > Check for Update). Otherwise you can download from Firefox 4 Beta site