We need some information. Are you using sync? Are you using the same computer? Are you using the same user account on the computer? Anything else you can tell us?

This happens when you both use the same Firefox Account to Sync data and have the bookmarks selected. When you use Sync then data from all devices that are connected to a specific Firefox Account is merged to other connected devices. Both of you either need to use their own Firefox Account on simply not use Sync. Sync is optional and is meant to sync data between other devices and isn't meant to be used for backup purposes.

I have not intentionally chosen "Sync" How do I unsync these 2 computers if this is my problem?

Shut down sync from here.

Type about:accounts<enter> in the address bar. [v57+] https://accounts.firefox.com/settings