Firefox will now 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 set to false to disable this feature.

• browser.tabs.unloadOnLowMemory = false

You can open the about:config 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

• 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/

Hello FireFoxFan1,

This sounds like one of the new features that came with the update to Firefox 67.0 :

See under "New" - suspending unused tabs

And also :

https://blog.mozilla.org/blog/2019/05/21/latest-firefox-release-is-faster-than-ever/

See especially the section :

Suspend Idle Tabs: You shouldn’t feel guilty about opening a zillion tabs, but keeping all those tabs open uses your computer’s memory and slows down its performance. Firefox will now detect if your computer’s memory is running low, which we define as lower than 400MB, and suspend unused tabs that you haven’t used or looked at in a while. Rest assured if you decide you want to review that webpage, simply click on the tab, and it will reload where you left off.

If you want to disable this feature :

Type in the address bar about:config and press Enter (promise to be careful, if asked)

Type in the search bar and look for the preference :

browser.tabs.unloadOnLowMemory

and set its value to false

Then close and restart Firefox.

Cor-el and I posted at the same time ......