The Firefox 4.0 beta for Mac, Linux, and Windows is now floating around online and brings 64 bit support for Mac OS X.
This beta is definitely not for the casual user as it comes with a number of stability warnings and will probably be quite buggy till it reaches public beta.
Windows – Firefox_Setup_4.0_Beta_1.exe
Mac – Firefox_4.0_Beta_1.dmg
Mac 64-Bit – Firefox_4.0_Beta_1.dmg
*This is beta version, so mainly for testing, We or Mozilla Foundation not responsible for using this.