Browser firm admits failing to spot malware.
Mozilla has admitted in a security notice that two experimental add-ons for its Firefox browser contain Trojans that affect Windows machines.
The firm has since removed the add-ons from its official pages, but estimates that around 5,000 instances have already been downloaded.
"Two experimental add-ons, Version 4.0 of Sothink Web Video Downloader and all versions of Master Filer, were found to contain Trojan code aimed at Windows users," said the company in a statement.
"Version 4.0 of Sothink Web Video Downloader contained Win32.LdPinch.gen, and Master Filer contained Win32.Bifrose.32.Bifrose Trojan. Both add-ons have been disabled on Add-Ons for Firefox."
Mozilla warned that users who have already downloaded the add-ons will become infected.
Simply installing the add-ons will execute the Trojan the next time Firefox starts, while uninstalling them will not eradicate the problem. The company advised the use of an anti-virus program to remove the malware.
Mac and Linux users are not affected.