This week's admission comes after an announcement last week that two add-ons for Firefox – Master Filer and Sothink Web Video Downloader – had been withdrawn after they were found to be infected by a trojan.
In a statement posted to its blog last night, Mozilla said: "We've worked with security experts and add-on developers to determine that the suspected trojan in Version 4.0 of Sothink Video Downloader was a false positive and the extension does not include malware."
The case has highlighted the fact that Firefox add-ons are largely developed by third parties to Mozilla, Infosecurity notes.
Coupled with the fact that Firefox is an open source application, meaning that thousands of volunteers are working on improving the program code, this makes the task of verifying the code from a company perspective almost impossible.
According to CNet's IT reporter Seth Rosenblatt, meanwhile, earlier estimates of 6000 affected Windows downloads have been revised to 700 downloads.
And the good news is that Apple Mac and Linux Firefox users were not affected by the malware problem.
In his report on the matter, Rosenblatt quoted Nick Nguyen, Mozilla's add-ons director, who said that the incident that uncovered Master Filer started when a user running an antivirus program from Eset generated a warning.
"All add-ons uploaded... are reviewed for malware with automated tools", said Nguyen.
"ClamAV failed to detect the Trojan in Master Filer which caused us to re-evaluate our toolset. After upgrading our process, we rescanned the remaining 58 000 files on AMO and detected one additional instance of malware", he added.