This addon was named ShiftLeftClickDownloader, but has many preferences. You can adjust parameters of mouse event, key modifiers, would you like download only links, only images or both, would you like to be asked what to download if a link and an image both were found or you prefer always download images and so on.
I was asked to create addon similar to Shift + LeftClick, that opens new tabs on shift + left click event and not new windows, only this addon will download files and images on prefered mouse event.
I'm not active user of this addon, so please report me for bugs if you want it to be improved.
There are to download methods available in this addon, but I recomend to use the first (default) one. The second uses native firefox files that is not part of stable API, so it can be broken in the future.
