I am using this addon v3.5 on Firefox v62 under Windows 7. This addon is great!
I have setting, "Above browser's native overlink panel (with no URL shown by default)", Offset from bottom: 16.
When I visit a page with maximized Firefox window and hover my mouse over a link, this addon perfectly displays last visited date just above the link text in the native overlink panel.
However, if I set Firefox window to normal state and resize it with smaller window width, a horizontal scroll bar appears in Firefox window. At this moment, if I hover the link in the page, the last visited date is not perfectly located just above the link text in the native overlink panel. There is a gap between them. So, the position is affected by the horizontal scroll bar.