Ideally, only tabs that are no longer needed will be unloaded and the user will eventually restart the browser or close unloaded tabs before ever reloading them. A natural metric is to consider when the user has last used a tab. Firefox unloads tabs in least-recently-used order.

Tabs playing sound, using picture-in-picture, pinned tabs, or tabs using WebRTC (which is used for video and audio conferencing sites) are weighted more heavily so they are less likely to be unloaded. Tabs in the foreground are never unloaded. We plan to do more experiments and continue to tune the algorithm, aiming to reduce crashes while maintaining performance and being unobtrusive to the user.