Shows just fullsize Image with hotkeys & without pop-ups on many image-hosting sites
It's Greasemonkey's problem, was working fine before, can't fix.
Just use the hotkey 'P' or configuration webpage - https://gist.github.com/Owyn/8553d7953d948228e312
Isn't it because of the changes in unsafeWindow:
https://blog.mozilla.org/addons/2014/04/10/changes-to-unsafewindow-for-the-add-on-sdk/
No. Not even using it.
OK.
Found the relevant GM-bug: https://github.com/greasemonkey/greasemonkey/issues/2139
Marking this thread as "good", as it's no problem of the script.
Opening the "Handy Image Configuration" doesn't work (2015.04.21) [NOT A BUG OF THE SCRIPT]
Opening the configuration-dialog using the Greasemonkey-menu doesn't work. There's no message or error shown in the Browser Console.
Bringing the dialog up with pressing P does work.
Handy Image 2015.04.21 (no other script installed)
Greasemonkey 3.1 (no other addon installed)
Firefox 37.0.2 (new profile)