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