Yes I want to save the image (picture file like GIF, JPG, PNG, etc.) to the disk. The files are always downloaded again, no matter whether I use the built-in download-manager (or whatever it is called, sorry I don't know its exact name) of FF or an add-on like DownThemAll.
Is the page cached as a whole, like an MHT or ZIP or some other archive file? Well, in that case, probably the image would have to be downloaded again since not all archive types allow individual files to be extracted. But I have one doubt. If that were the case, then I must be able to save a complete page instantaneously, which is not the case. As you can check, saving the complete page again initiates the download of each individual element (pictures, HTML files, CSS, JS, etc.) again. Why is that the case if the whole page is already cached?
Maybe this is again due to the fact that the users normally wanna save the page in HTML format (with the other elements like images etc. as separate files), and not as a whole mass. But then, it's a very bad strategy on the part of Mozilla (if it really caches the whole page as a single file, which I don't know). No other browser caches the page as a whole.
Sorry if I went off-topic. I hope I provided the info that you asked for. Please ask anything again if I'm not clear. Thanks.