Based on Linkify Plus. Turn plain text URLs into links.
If the script lacks of updateURL / downloadURL, Greasemonkey will check the original location for further updates: https://greasyforks.org/en/help/rewriting
Greasemonkey uses Firefox builtin update system. If you have turned off the auto update, it might be the problem.
Probably doesn't auto-update
The script doesn't seem to update on its own. It's probably because there is no updateURL / downloadURL defined in the script.
The script works pretty well otherwise.