Step #1: Disconnect the old Firefox from your Firefox Account. You can do that from the main menu. Then any local changes will not be synchronized. Step #2: Create a new profile for your friend. Here are the steps: 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 Default2022, ignore the option to relocate the profile folder, and click the Finish button. Firefox should switch the default profile to Default2022, so the next person who starts Firefox will start up in that non-personalized profile. Let's test that. Exit out of Firefox and start it up. Does it look like a new non-customized installation? Assuming so, perfect. Now let's proceed to: Step #3: Delete your personalized profile(s) Inside Firefox, type or paste '''about:profiles''' in the address bar and press Enter/Return to load it. For each profile that has a Remove button, click that button and use the option that removes the data folder. This should purge your local data folders bypassing the Recycle Bin. That's it. Well, that doesn't delete downloaded files from wherever they are saved, but you can check for those in Windows File Explorer.