Firefox has been continually gaining market share against main competitor Internet Explorer since its debut four years ago, which may be one reason why malware authors are looking for new avenues to infect computers, Canja said.
Users could be infected with the Trojan either from a drive-by download, which can infect a PC by exploiting a vulnerability in a browser, or by being duped into downloading it, Canja said.
When it runs on a PC, it registers itself in Firefox's system files as "Greasemonkey," a well-known collection of scripts that add extra functionality to Web pages rendered by Firefox.
BitDefender has updated its products to detect it, and other vendors will likely follow suit quickly, Canja said. Users could avoid it by only downloading signed, verified software, but that's a measure that restricts the usability of a PC, he said.
The malware is not present in Mozilla's repository of add-ons, Canja said. Mozilla had taken steps to ensure that its official site hosting add-ons -- also called extensions -- are free from malware.
In May, Mozilla acknowledged that the Vietnamese language pack for Firefox contained a bit of unwanted code. Although widely reported as a virus, the language actually contained a line of HTML code that would cause users to view unwanted advertisements.
Mozilla now scans new add-ons for malware. However, those scans will only detect known threats, and there was no signature in the security software Mozilla was using at the time that could detect the code.
Mozilla said the code probably ended up in the language pack after the PC of its developer became infected. More than 16,000 people downloaded the language pack, but only about 1,000 people regularly use it.
After the incident, Mozilla said it would scan add-ons in its repository when antivirus signatures were updated.
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