Finding out which profile is the one used when there are many in the %Appdata%\Mozilla\Firefox\Profiles folder:
For each profile, check the last modified date on history.dat (the one they used last should be the one that was most recently modified) OR check bookmarks.html (right click open in IE if Firefox isn't working) to make sure that it has all the right bookmarks.
If they created a profile with you or by themselves... the created date for the FOLDER will tell you that.
If none of the last modified dates in the profile are recent, it's possible that they made their own folder in a non-default location. Start profile manager (firefox.exe -profilemanager) and mouseover each profile to see its location (or at least the first part of it).
Last resort: Open profiles.ini (%Appdata%\Mozilla\Firefox) in a text editor which will have the profiles and their paths.