This is not a definitive answer. I am not even a Mac user. Official Mozilla Firefox downloads have As Far As I Know never contained malware they have however occasionally triggered false positives, which I imagine is what is happening here. Also if that malware is the same as https://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2013-3860 it looks like it is something that affects only Windows so it should not be any immediate problem on a Mac. The official download site would be * getfirefox.com * or direct from Mozilla https://www.mozilla.org/firefox/all/ You would presumably have initially got an update using the installed Firefox, there is just a possibility that is corrupt (or settings are corrupt) If you wish to you could try what we call a clean reinstall where the original application files are trashed before reinstalling. However from what you say I presume the problem is only detected after the new Fx48.0.1 is installed and that your download would have been from an official site as you said. * [[Troubleshoot and diagnose Firefox problems#w_5-reinstall-firefox]]'''_5-reinstall-firefox''' ** (You should be seeing the Mac version of the article [/kb/troubleshoot-and-diagnose-firefox-problems#firefox:mac:fx48] )