I've installed the Greasemonkey Add On but I can neither install any scripts nor I can create my own scripts. When I click on the Add On in the upper menu bar I see following entries:
When I click on "New User Script ..." nothing happens, just sometimes it opens like 5 tabs of an empty script.
I am using Firefox 57.0.2 (64 bit) and Windows or Ubuntu (latest versions)
Same behavior on both Operating systems
Any ideas how I can fix this?
Use Violentmonkey or Tampermonkey instead of Greasemonkey.
Greasemonkey 4 is severely crippled and, frankly, Greasemonkey has been lagging ever further behind Tampermonkey in terms of capabilities and ease -- even before this major break.
In Fact, the Greasemonkey team itself recommends that you switch to another engine:
...but this will break some scripts. If you rely on such scripts, you might want to install Violentmonkey or Tampermonkey, both of which provide better compatibility for existing scripts.
Violentmonkey is good, if a little unpolished and incomplete. And it's open source.
Tampermonkey is better, and has bested Greasemonkey in ease-of-use and capability for a few years now. But it's not open source and does more communication with the "mothership". (Nothing nefarious as far as can tell.)
Tampermonkey is also ported to the most number of other browsers, and has easy import/export tools.