Finding a replacement for FireGestures is going to be a bit tricky. Of the various options out there, it's a tie between this one and Foxy Gestures.

This one almost provides everything I need, with one exception. It lists the option as 'Save Image As' but it's not quite the same as 'Save Image As...' in the pop up context menu.

For one, the 'save image' gesture always defaults to Downloads, and that's not always where I want to save an imagine. For another, Firefox has this weird way of remembering where things were previously saved and this doesn't quite seem to work with it.

The 'Save Image As' tends to remember where things were saved based on URL - partially. (For when it's just a picture and nothing else, Save Page seems to work better). I'm hoping that one of the replacements for FireGestures can manage to emulate that option in a better way. Save Page doesn't seem to do anything that I can tell. It doesn't bring up a prompt to save the page, or to bookmark, or anything else. Maybe I don't fully understand it.

It's also possible this is just a straight up port from Chrome and it's not meant to be fully functional with FF56.