If I save any image that Firefox has already downloaded, it starts downloading the image again, which is dumb and a waste of bandwidth.
Is there a way to prevent this? I'm using FF version 33.
The Image Picker Add-On will do this.
https://addons.mozilla.org/en-US/firefox/addon/image-picker/
Once enabled, it will cache images on first download, then it will download from the cache on all future downloads.
It also has a few other features you might like.
Cheers!
Sources: Personal Experience