No, SecurityDefender is malware. Avast did the right thing by blocking it. Here is a page with more information: http://www.bleepingcomputer.com/virus-removal/remove-security-defender

Or you can do a google search for "SecurityDefender".

Firefox can not detect trojan programs. Be carefull not to install a fake virusscanner, you will be infected with malware instead. Personally I would not trust this Security Defender thing, it sounds like a scam to me. I guess Avast is stopping the download because it is a virus. In general it is always very dangerous to install software that you get by mail. Also never install a fake "Firefox-update" that is send by mail.

Security Defender is malware posing as anti-malware.

Read and understand all of the following article.

• If you can not access the article or can not download the recommended removal software on the infected computer, then you must go to a non-infected computer to do so and transfer the recommended removal tools to the infected computer via CD/DVD or on a USB/thumb/pin/jump drive.
• See --> http://www.bleepingcomputer.com/virus-removal/remove-security-defender

Security Defender is not associated in any way with Mozilla or Firefox. It is an infection that you picked-up on the internet by downloading/installing something, by clicking on an e-mail link or through a "drive-by" download which required no action on your part. Keep your firewall and anti-virus/anti-malware application(s) updated; they are your first line of defense.

Firefox will protect you from known malware and phishing sites, but new ones are created every day and it takes a while to discover the new sites. Some malware sites are created and then disappear within a matter of days. Some legitimate sites become infected with malware or malware links and the owners of the sites do not discover the problem for some time.

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