Third party credentials showing in Saved Passwords

That would only happen if those peoples have used your computer or if you are using Sync and connected to Sync on a computer that is not your own. You should never connect to Sync on a device that is not your own as Sync works both ways: your data gets on their device and their data gets on your device.

I never used sync on a device that's not my own.

I never let anyone use my devices and have not screen shared.

I do not recognize the usernames or websites.

Any other way this could happen?

mvuister, some thoughts. do you use any add-ons, especially ones that deal with passwords or forms?

Have you ever manually imported or restored passwords from a file?

The devices that you use, are they new or second hand?

The only other possibility I can think of is that you visited a site that offered these credentials at its end in one of its form fields, you took some action that Firefox interpreted as you entering a new login, and so saved them in your passwords list? (Although you should still have received a notification asking you if you wanted to add these new credentials, which you would have had to have accidentally accepted.)