Check out these pages... https://support.mozilla.org/en-US/questions/1360041 https://hacks.mozilla.org/2021/05/introducing-firefox-new-site-isolation-security-architecture/ https://billmccloskey.wordpress.com/2013/12/05/multiprocess-firefox/ You can set fission.autostart = false on the about:config page to opt-out of this experiment and disable Fission. You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue". https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Make sure you have the first setting on the General page of Settings unselected ("Open previous windows and tabs").