Hi

Thank you for your feedback. This is something that our developers are aware of and are looking into. You can follow progress on this and add comments at:

https://github.com/mozilla-mobile/fenix/issues/16859

This must not be actively worked on since the issue was first reported on December 4, 2020.

You can't change this behaviour, but you can go to Settings -> Tabs and select Close Tabs after a week/month and enable Move old tabs to inactive.