Does this occur on any private browsing tab or only when visiting a particular site in that private browsing tab? Firefox by default does not do that. It does remind me of a addon called Snooze Tabs though. https://addons.mozilla.org/en-US/firefox/addon/snoozetabs/ Check your [https://support.mozilla.org/en-US/kb/disable-or-remove-add-ons#w_how-to-disable-extensions-and-themes Addons Manager] area to see if you have this addon or a similar one. Disable all of them or any ones you suspect of having the power to do this. If you find out a addon was the cause of this, please report back! I'd love to know so I can pass this tip onto anyone else dealing with this weird problem. Thanks!