IIRC, the developer initially resisted shifting to WebExtensions and then eventually (my assumption) half-heartedly did convert his extension to it, but in the meantime TamperMonkey developer got to it much sooner and worked hard for making it work equally well in both Chrome and Firefox. It is still not as feature rich as TamperMonkey and not even actively maintained anymore I think.
Wasn't this a casualty of the XUL addons being removed? The other monkey extensions were ported to the new Web Extension APIs.
Greasemonkey still works in recent Firefox versions.
It does, now, but its developer took a really long time in porting, in that time TamperMonkey was developed and had really gained popularity.