I was using latest version of Firefox 3.6.3,  but wanted to install a add on which works only on Firefox 3.5.  But I didn’t want to uninstall my latest Firefox. 

Solution for this was to

  • Install  Firefox 3.5 into a different folder from the existing  Firefox installation. 
  • Then I started the Firefox 3.5 with -P option, to  get the Profile dialog and created a new profile for  Firefox 3.5. 
  • Then started Firefox 3.5 with the new profile.