Use these links to create a new profile.

http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Type about:profiles<enter> in the address bar.

One your new profile is set up, you can remove the old one.

I can spell that out in a more step-by-step manner.

First, if a dialog pops up for a password, go ahead and cancel it.

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Click the Create a New Profile button, then click Next. Assign a name like March2018, ignore the option to relocate the profile folder, and click the Finish button.

After creating the profile, scroll down to it and click the Set as default profile button below that profile, then scroll back up and click the Restart normally button. (There are some other buttons, but please ignore them.)

Firefox should exit and then start up using the new profile, which will just look brand new. Hopefully it runs smoothly?

To delete your son's profile, load the about:profiles page once again and you should see a Remove button for it.

thanks so much all of you