This would enhance usability greatly. It was possible with FLST add-on long time ago. Keyboard shortcut (ctrl+tab) is not as easy.
Also closing current tab should take you back to previously viewed tab. This is luckily still possible with Select After Closing Current add-on.
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Most definitely yes. Having stayed with Firefox 56 for a long time in order to keep old add-ons alive, and then spent another couple of years with current Firefox, this is one of the few things that remains missing.
In terms of rationale/use case: I believe that as many actions as possible should be achievable under the current input mode. That is to say, if your hand is on the mouse, you should not be expected to locate the keyboard unnecessarily, and likewise if your hands are on the keyboard, you should not be expected to locate the mouse unnecessarily. Ctrl+Tab in last-visited order is perfect when you are using the keyboard, but if you need to glance at a tab and do so with the mouse, by far the most convenient way to return to the previous tab—and in full compliance with Fitts’ Law—is to click the same tab again. This avoids needing to hunt along the tab bar to find the tab you were in before (since you know you forgot to make a mental note of where to find it again) and avoids needing to move to the keyboard for Ctrl+Tab.
I have no objection to this requiring an add-on, so long as one were to exist and that it is possible for an add-on to achieve this.