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Mozilla last night announced that two experimental Firefox add-ons, Master Filer and the Sothink Web Video Downloader version 4, infected victim PCs with Trojans when either add-on was installed.
via Malicious Firefox Add-ons Installed Trojans – PCWorld.

Software Firms Fear Hackers Who Leave No Trace – NYTimes.com.
If hackers could steal those key instructions and copy them, they could easily dull the company’s competitive edge in the marketplace. More insidiously, if attackers were able to make subtle, undetected changes to that code, they could essentially give themselves secret access to everything the company [...]

Security Fix – What To Do When Scareware Strikes.
First thing: Take a deep breath and don’t panic!