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Mozilla plans to introduce a new feature in Firefox 67 Stable that aims to improve the browser's memory usage in low memory conditions.

If things go as planned, Firefox 67 will introduce a new feature to unload unused tabs to improve memory. The initial bug report dates back eight years but work on the feature began in earnest just a short while ago.

Mozilla plans to unload tabs in Firefox in low-memory situations to reduce the number of crashes that users experience caused by low-memory. The bug lists another scenario, to free up resources, but it is not clear yet if and how this will be implemented.

browser.tabs.unloadOnLowMemory


Mozilla uses a simple priority list to determine which tabs to unload when the event fires (from lowest to highest)
  1. Regular Tabs
  2. Pinned Tabs
  3. Regular Tabs that Play Audio
  4. Pinned Tabs that Play Audio
The feature is already available in Firefox Nightly. It was turned on by default on my system but you can control it with the preference browser.tabs.unloadOnLowMemory.

True means the feature is enabled, False that it is disabled. It appears that it is available on Windows only at this point because its the only platform that Mozilla can detect low-memory conditions on according to the bug assignee Garbriele Svelto.

Firefox 67 will be released on May 14, 2019 to the Stable channel of the browser according to the release schedule.