This is a great feature. I'll freely admit to taking gratuitous screenshots to share things (Cmd-Shift-Ctrl-4 makes it so easy on a Mac); of course, this makes the image impossible to search for later. It would be great to be able to search through the images I lazily take with a good old text search.
If Mozilla keeps it up with features and performance improvements, they'll lure me back from Chrome.
Screenshots is implemented as a WebExtension. Unfortunately, the WebExtensions framework in Firefox currently appends items to the bottom of the context menu--so you'll notice this with other WebExtensions, too.
There's a bug open to change this behavior: http://bugzil.la/1325758 . If you'd like to hack on some browser JS, I'd be happy to help get you connected with a bug mentor on the addons team :-)
Some of the use cases for precise placement capabilities seem pretty compelling, but here's hoping for accessible user customization options as a long term solution, perhaps in conjunction with the ability for WebExtension developers to set appropriate default placement, as long as that placement is not below Inspect Element. Inspect Element at the bottom is sacred!
