It is actually exposed, since a few versions ago. It's in the Preferences under ...

_phaq on May 10, 2018 | parent | context | favorite | on: Firefox 60 released

It is actually exposed, since a few versions ago.

It's in the Preferences under General -> Tabs -> "Ctrl+Tab cycles through tabs in recently used order". Right in the middle of the screen, when you open the Preferences-page...