Firefox will detect if your computer’s memory is running low, which is defined as lower than 400MB, and suspend unused tabs that you haven’t used or looked at in a while. The new tab unload feature is controlled on the about:config page with a preference that You can control this feature via a pref on the <b>about:config</b> page. *browser.tabs.unloadOnLowMemory = false Note that Firefox might crash if you are really too low on memory if you disable this feature. You can open the <b>about:config</b> page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue. *http://kb.mozillazine.org/about:config <hr> *http://www.ghacks.net/2019/03/01/firefox-67-automatically-unload-unused-tabs-to-improve-memory/ *https://blog.mozilla.org/blog/2019/05/21/latest-firefox-release-is-faster-than-ever/