Hello Blaž, probably false alarm ! If you download from the official mozilla.org then no trojan or rootkit or other virus at all.

if you like to check it you can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• Anti-Rootkit Utility - TDSSKiller
• AdwCleaner (for more info, see this alternate AdwCleaner download page)
• Hitman Pro
• ESET Online Scanner

thank you

More than likely a "false positive". But once you have Firefox 43.0 installed and updated to Firefox 43.0.1, you don't need either of those Setup files - just delete them. Their only use is to get Firefox installed.

Thank you from all my heart for such a quick response. You made my day much nicer... I was worried how it's possible that on official mozilla site infected installer. I thought it's impossible, but now when you told me it's impossible, I'm calm. I must admit I was terrified :)

I also checked registry changes which "Variant.Kazy.kx" makes and files it installs and there are no traces of "Kazy". I'm only curious how this false positive can happen, but this is another topic for another forum.

And on this opportunity I want also give you congratulations for your work on Mozilla web browser. It's fabulous. I use it many years and it's my N1 browser.

Thanks again for your time and have a nice following holidays! Blaž

Some antivirus and malware scanners can still have false positives on the stub installer but not the full setup on Windows even though the stub option has existed since Firefox 18.0

The full setup for Windows is at www.mozilla.org/firefox/all and is the Systems & Languages link below the Free Download button on www.mozilla.org