How do I install userscript from the local machine with GreaseMonkey 1.12 (2013 autumn)?
Found the way!
You are expected to drag-n-drop (from a file-manager window of your choice) the userscript file into the greasemonkey "manage userscripts" settings pane in about:addons in the browser. :)
(I could not drop them on the toolbar button, however.)