How can I change the default location of profiles.ini?
Is there a way to change the location FF looks for the profiles.ini file?
Why do you want to do that?
It is not possible to change the location of the profiles.ini file. The profiles.ini file is always in the default location for the profiles in AppData\Roaming\Mozilla\Firefox. The profiles.ini is only required to find the profile folder if you open a link in an external application in the default browser. In a shortcut you can use an absolute path to the profile.
Um, just because? For one thing, it would allow me to run isolated instances without contamination.
What kind of contamination are you talking about?
You can always use the Profile Manager to create extra profiles, but the Profile Manager will always show all the profiles that are registered in profiles.ini.
You can add -no-remote to the command line to open another Firefox instance with its own profile and run multiple Firefox instances simultaneously.
If you meant that you don't want Firefox to put data on the C drive, consider using a PortableApps build. By design, it contains itself to a single folder in the location of your choice. (Plugins still might use the Windows TEMP folder, etc., however.)
It's not an official release, but it has a long track record.