From MozillaZine Knowledge Base

When downloading a file you may find that Firefox launches an unwanted blank window. This occurs because the webmaster has specified that the download should start in a new blank window even though this was probably not their intention. This is a known issue, Bug 241972.

You can fix the problem by downloading the Disable Targets for Downloads extension. Or the TargetKiller extension.

Webmasters can fix the problem on their site by removing target=_blank or window.open from the download links.

External Links

• Disable Targets for Downloads extension homepage
• TargetKiller extension homepage