This extension worked great for a long time, only showing the 404 detected popup when I actually encountered a missing page. However, I recently updated from Firefox 56 to Firefox 63 (which may have updated the extension too), and that caused the extension to start detecting spurious 404s, as other reviews describe. When this extension works, it is a convenient way to open the WayBack Machine, but I’m going to have to disable the extension for now until it gets fixed.
On older Firefox versions I used to use the add-on Resurrect Pages by Anthony Lieuallen. I stopped using it because it became incompatible with my Firefox version, but apparently it supports the latest Firefox version again. It may be worth checking out.