The feature itself is available in Firefox 87 as well. A new prompt message will appear after opening a tabs view. There you can see some additional settings in the tabs settings menu. The promoted option allows you to select a time when opened tabs will be closed automatically.
This feature can save you some time by killing pages you don't need anymore as well as saving some device resources that would be needed to keep them open.
Available options are: Manually, After a day, a week or a month.
How to test:
- Get Firefox Nightly v89.
- Tap on the Tabs button at the bottom.
- See a new prompt and tap on View Options.