The addons has been downloaded several thousands of time over past 3 months. Mozilla warned that a malicious program inserted adware code into a Firefox plugin. According to Mozilla security chief Window Snyder:
Everyone who downloaded the most recent Vietnamese language pack since February 18, 2008 got an infected copy. Mozilla does virus scans at upload time but the virus scanner did not catch this issue until several months after the upload.
=> Virus found in Vietnamese language pack (Via Y! news)