I’d noticed recently that whenever I saved something from Firefox on the Mac, it’d pop up an empty download manager and put an oddly named file on my desktop (for instance, ‘f837fed.exe’). I wondered if this might be something odd with an extension, or even worse did I have a trojan somewhere (but this is a Mac, can’t be one of them!).
I got a bit peed off with it today and finally got to the bottom of it after attempting a fresh install (which didn’t work). It turns out that if you delete the directory where Firefox puts its downloads, it’ll go a bit mental and spew weird partially-downloaded files onto your desktop; there’s something on the Mozilla lists about it here.
The solution isn’t a reinstall, but instead ignore what it says in Firefox’s Preferences->Main screen and re-set your “Save files to…” directory to be the desktop. Lo and behold, it started working fine, I can drag-and-drop images back into Photoshop, and the world’s a better place.