Sound s like it is a common problem - I did a little searching and found these suggestions. For Macs.

[Firefox -> Preferences -> Main Tab
Change the location where you save the files to another location. See if that works. If it doesn't change the other settings in regards to the downloading section.]

another suggestion is to make your browser window/or tab, a bit smaller, then just drag and drop the picture into a folder or onto the desktop.

and here is the "save Image" Mac addons for FF -
https://addons.mozilla.org/en-US/fir...eeklydownloads

one of those might be a fix for you.