Use the profile manager to set the default profile that you want to use and tick the box to always use that choice. After having set that you need to revert from using the Profile Manager and only use desktop shortcut that specifies the profile to use to avoid changing that setting.

The default profile set in profiles.ini is used when you click a link in an external program and Firefox isn't open yet. Be careful with using the -no-remote switch as that makes that Firefox instance invisible to links clicked in external programs.

http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile

Also, the Profile with the line Default=1 in the profiles.ini file is the Profile that Firefox will use as the "default" Profile for external programs; the "name" of the Profile has nothing to do with it.

Also be sure that StartWithLastProfile is set 0 in profiles.ini so firefox does not change default profile every time shuts down.

[General]

StartWithLastProfile=0

You will need to open "profiles.ini" in Wordpad or Notepad and edit it. Please note that I am runnig Windows 7 and Firefox 11. On my computer, the path to my profile.ini file is: "c:\Users\Lyndhurst\AppData\Roaming\Mozilla\Firefox". When you open the file you should see your profiles (1 or more) and the profile that opens first has an extra line that says "Default=1". You need to move that line to the profile that you want to open first, either by using "cut and paste" or by deleting it and re-typing it. Hope this helps.