You can use the Public Fox add-on to password protect certain parts of Firefox, though the protection is not too difficult to bypass if a person knows about how Firefox works.

...though the protection is not too difficult to bypass if a person knows about how Firefox works.

For example, Diagnose Firefox issues using Troubleshoot Mode can be used to bypass it.

If it is only specific settings that you which to protect then you can lock the related prefs. If your daughter doesn't have write to the Firefox program folder then it is impossible to change such locked settings. Of course it won't help if the portable Firefox version is used.

See:

http://kb.mozillazine.org/Locking_preferences
http://kb.mozillazine.org/about%3Aconfig