I noticed that when I have been using Firefox, very often when I would right-click on an image and choose “Save Image As” the dialog would simply disappear instead of bringing up the file folder and letting me save the image. This got to be rather irritating after a while. I started doing some troubleshooting and disabled all of my add-ons. The problem also disappeared, indicating that one or more of my addons was likely the cause.
I re-enabled and started looking at ones that were image related. I discovered that one of my favorites “Image Zoom” was the culprit. If I disable it, I can right-click and Save Image As dozens of times with no problem. If I re-enable it, the Save Image As dialog will sproadically disappear as soon as I click Save Image As.
For now, I’ll leave it off, though it really has been a great add-on.