In Firefox, you can drag any images to the desktop or anywhere in Finder and it will download it. I turned on Mouse Gestures, and with that on, dragging of the mouse starts a gesture and won’t trigger the default action. To get both gestures and drag-and-drop to work with the left button, i.e., just one finger touching the trackpad, I switched from Mouse Gesture to All-in-One Gestures, and configured it like this:

Now navigation gestures as well as drag-n-drop work perfect.