To answer my own question: it doesn't, at least not on my system. I can see the Firefox child process count maxing at around 10 processes with the default fiss

To answer my own question: it doesn't, at least not on my system. I can see the Firefox child process count maxing at around 10 processes with the default fission.autostart set to false: > ps aux | grep "firefox.*childID" | wc -l > 10 Also the about:processes page shows some processes containing 'Shared tabs'. As soon as I flip fission.autostart to true it creates proper separate processes for each tab with the above ps aux command increasing the count as I open up tabs.