This addon adds a "Download" button to images viewed in their own tab (NOT inside web pages). By clicking it, the image being displayed will be downloaded, without re-downloading it from the server, thus, fast.
Also, if the image 404'd on the server, or, for any reason, can't be reopened, you WILL be able to download it, if it's open on a tab.
If you wish to change something, submit an issue or pull request on the github page:
https://github.com/leogcurvelo/FirefoxFastImageDownload