From MozillaZine Knowledge Base
The profiles.ini file contains information used to keep track of profiles in Firefox, Thunderbird, and SeaMonkey (2.0 and above). Whenever you use the Profile Manager to create, rename or delete a profile in these applications, the changes are reflected in the profiles.ini file. The profiles.ini file is located inside the "Firefox" "Thunderbird" or "SeaMonkey" folder in the profile folder path for each application (see Show hidden files and folders).
On Windows: You can use "Start -> Run" or press [Windows key]+[R] to open the Run box, then type in one of the following and click "OK" to quickly open the appropriate folder:
Since the profiles.ini file is a plain-text file, it can be easily opened, viewed and edited, if necessary, with a text editor such as Notepad. Caution: An "already running but is not responding" error can occur as a result of incorrect editing of the "profiles.ini" file (bug 278860). Always make a backup copy before editing the profiles.ini file.
If you delete the profiles.ini file, a new default profile will be created the next time you start the application and a new profiles.ini file will be generated that references the new profile. The Profile Manager will only show the newly-created profile and the old profiles will no longer be listed. For information about adding profiles to the Profile Manager or accessing profiles that are no longer listed, see the articles, Moving your profile folder and Starting your Mozilla application with a specified profile.
Here are two examples of the profiles.ini file contents :
IsRelative=1 refers to the expected or "default" folder path that would be relative to the operating system.
IsRelative=0 refers to a custom profile location such as Path=D:\Mozilla\Firefox\Profiles\MyProfile
StartWithLastProfile=1 indicates that the "Don't ask at startup" option is selected.
Default=1 refers to the last selected profile.