Malware detection has been added to the Mac and Linux versions of Firefox. Firefox asks Googles Safe Browsing service if the file is safe by sending it metadata. If the file contains malicious code, the download manager will block access to the file until the user performs a right-click, and unblocks it manually. In addition to this, Firefox 40 now issues a warning if you visit a page known to contain deceptive software that can make undesirable changes to your computer. You will be presented with the following warning if such a page is known to be dangerous.”]
Source: https://blog.mozilla.org/security/2015/08/11/expanded-malware-protection-in-firefox/