who can see my private tabs

Private windows only hide those tabs from other people who use the same Firefox profile later: they are not saved in history, and cookies/site data from that private session are cleared when the private windows close. They do not hide the sites from the website itself, your network/admin/ISP, or someone who can look at your screen while the tab is open. If the risk is another person using your device, close all private windows when finished and use a separate OS account or device lock; private browsing is not a strong access-control feature.