I have been finding Firefox is unable to download files lately. Below is a fix you can make to the about:config file to resolve this.
Bypass Windows Security Policy check
Starting in Firefox 3.0.6 and in Firefox 3.5.x you can bypass the Security Zone Policy check. by using about:config to add a new Boolean preference, browser.download.manager.skipWinSecurityPolicyChecks and set it to true. [44]. This preference was removed in Firefox 3.6. [45] [46]
An alternate solution is to reset your Windows Internet security settings, as explained below. In Firefox 4 and above, up until recently, you could bypass the Windows security policy check by setting the preference browser.download.manager.scanWhenDone to false in about:config. [47] Note, however, that this preference is no longer available in current Firefox versions.
Source: Unable to save or download files - MozillaZine Knowledge Base
