Firefox accuses of malware, but virustotal have 0 matches

Question

I sometimes download PDFs on libgen and sometimes Firefox will acuse them of being malware with the advice "this file contains virus or malware" but gives no details. I've run several of these files through virustotal and none give any problems in any of the scanners.

Some of them are djvu which I presumed was only a format to hold static images with no interaction, so it'd be much harder to host a malware. So is there a way to know what is firefox finding in those files?

Is firefox somehow better than all the antivirus tested on virustotal?

If someone wants to test some file please let me know so I can send the link

Answer 1

Firefox is not an anti-malware scanner, it uses a blacklist which it updates from various feeds e.g. Google safe browsing, phishtank, etc.

Since Firefox is pretty new in such a field, they tend to be "excessively paranoid", e.g. based on many bad URL and derive the blocking result, as mentioned by @Steffen Ullrich. As a Firefox user, you can help by clicking the button on the warning screen. When there are enough clicks, hopefully, Firefox will notice it and unblock it.

Unlike malicious file detection, URL blacklisting is never easy to do it "correctly", due to the dynamic nature.