I was trying to install Firefox 3.0b5 in OS X with Firefox 2 installed and it didn’t open both at the same time after some look around this was what I had to do:

  • Download the latest version of Firefox 3;
  • Install the application somewhere but your applications folder (ex. Desktop);
  • Rename the application of Firefox to something else (ex. Firefox3);
  • Copy the application to the Applications folder;
  • Open a terminal and run the following commands:
    • cd /Applications/Firefox.app/Contents/MacOS/
      ./firefox-bin -ProfileManager
  • The profile manger will appear, create a new profile and press “Start Firefox”;
  • Open Firefox3, choose the new profile and browse!