Works best when visited links are distinctly colored, such as with the extension:
https://addons.mozilla.org/en-US/firefox/addon/gray-visited-links/

Animated screen recording:
https://giphy.com/gifs/1Ya3O25ThPpUVCOG44/fullscreen

Install

• License: MIT License
• Weekly downloads: 2
• Average daily users: 47
• Rating: 5/5 of 5 ratings
• Created: 2018-03-19 05:48:20
• Last updated: 2018-03-19 19:20:07
• Homepage: https://github.com/rotemdan/LastVisitedTooltip
• Support site and email
• Orig: https://addons.mozilla.org/en-US/firefox/addon/last-visited-toolip/
• API: {16615320-157b-4e6b-b767-c7f5210792bb}