Why can't I add script with the file:// protocol anymore?

[Epik]

I found here https://stackoverflow.com/questions/19910568/can-greasemonkey-work-with-the-file-protocol that I should change the boolean value of the
extensions.greasemonkey.fileIsGreaseable
to true but the key doesn't exist anymore in about:config.

Browser Firefox 57.

[Evgeniy]

As I understand it, file:// protocol is not supported as the new WebExtensions API doesn't allow an extension to access file system.
You need web server which serves your .user.js files with "Content-Type: text/javascript" header to install them into FF.
Some info on that is here: https://wiki.greasespot.net/User_Script_Hosting .

[Anthony Lieuallen]

https://github.com/greasemonkey/greasemonkey/issues/2612

Maybe again in the future, now it's not supported.