The installation process informs the user that the add-on "requires permission to: "Access Browser Tabs" when asking whether to proceed with the installation, but it does not go ahead with adding the permission itself with out asking anything other than whether to install the add-on, it does not ask whether it should add the permission on behalf of the user, "Access Browser Tabs" is not among the set of permissions that can be enabled or disabled under. Settings-Apps→Permissions, and as for as I can tell there is no mechanism on my Samsung Android 9 tablet for providing that permission. I suspect that absence of this permission is the reason that no undo takes place and no closed tabs are displayed as undo candidates (or as anything else, for that matter).

Obviously this has worked on some systems ( it could not have earned so many stars in the past, otherwise), but there is some assumption that has been made about the way permissions are set that has ceased being true for at least some recent versions of Android (including the Android version running on my tablet (manufactured mid -2019 and updated mid-May to the March 2020 software revision).

Edit: I forgot to mention: I've been running an up to date version of Firefox, updated today to 68.8.1