As part of our commitment to help Firefox users stay safe online, we have recently expanded the malware detection features in Firefox. Thanks to new developments in Google’s Safe Browsing service we are now able to identify malware downloads in all of our supported platforms as well as warn users about potentially unwanted software.

The first of these changes, introduced in Firefox 39, consists of extending the monitoring of malicious file downloads to the Mac and Linux versions of Firefox.

When downloading a file of a type that usually contains Windows or Mac executable code (for example, .com, .exe, .msi, .app, .dmg) Firefox asks Google’s Safe Browsing service if the file is safe by sending it some of the download’s metadata (file type, name, size, hash, URL, locale). If the file is flagged as harmful by this service, 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 you encounter such a page:

While we believe that malware protection is in the best interest of all of our users, we recognize that some will prefer not to send any data about downloaded files to Google and hence provide an easy way to disable this feature.

https://blog.mozilla.org/security/2015/08/11/expanded-malware-protection-in-firefox/