Mozilla warned Wednesday that a malicious program inserted adware code into a Firefox plugin that has been downloaded thousands of times over the past three months.

Because of a virus infection, the Vietnamese language pack for Firefox 2 was polluted with adware, Mozilla security chief Window Snyder said in a blog posting. “Everyone who downloaded the most recent Vietnamese language pack since February 18, 2008 got an infected copy,” she wrote. “Mozilla does virus scans at upload time but the virus scanner did not catch this issue until several months after the upload.”

Mozilla is now going to add additional scans of its software to prevent this kind of thing from happening in the future, she said.