If you cancel the download in Firefox, it is likely that the downoad will still continue in the background. The observed bug was in Firefox 30 (Mac 10.8.5). To manually remove the download session, folloe the instructions below.

Mac OS X 10.7 and above: By default, Mac OS X Lion hides the ~/Library folder (which contains the profile folder, Crash Reports and other user data for Mozilla applications). You can open your user Library folder by holding down the Option key while opening the Go menu in Finder, then selecting Library. Once it's open, you can drag the small icon of a folder in the header of the Library's window into the Favorites section of the Sidebar, where it will be conveniently accessible whenever you want.

In this case the problem was caused by sessionstore.js in the Profile_Folder.

Removal of sessionstore.js fixed the issue for this session.