Rated 4 out of 5 stars
Updated script per instructions below from j917863 and Slideshow works fine on ver 60.3.2. Windows 10
Opened the renamed zip file with 7zip and edited install.rdf with Notepad.
Removed the old add on
Installed by clicking the Gear symbol next to the search window under Tool>Add-Ons>Extensions and choosing Install Add-on from File
Note if you click on Tools>Add-on Options>SlideShow it will cause Thunderbird to freeze requiring the use of Task Manager to close the program.