You can ignore the Wacatac.B!ml trojan if you downloaded Firefox from the Mozilla server as we have seen more reports about Microsoft Defender reporting this trojan and this is likely a false positive. If you didn't download Firefox from the Mozilla server (what you always should do), but clicked a link on a webpage or in a pop-up then you should remove this file to avoid getting infected.

• https://support.mozilla.org/en-US/search/?product=firefox&w=2&q=Wacatac.B!ml
• https://www.google.com/search?sa=N&num=100&q=Wacatac.B!ml%20windows%20defender

Where did you download Firefox from?

Downloading Firefox setup from mozilla.org like say https://www.mozilla.org/firefox/all/#product-desktop-release does not have random numbers as part of file name as it would be say Firefox Setup 117.0.1.exe

James I haven't downloaded Firefox for years. This was Firefox itself performing a regular update. I'm running Firefox 102.15.1esr (32-bit).

Official automatic updates don't use a setup executable or place anything in the Downloads folder.

Yes the internal updates in the official builds of Firefox for Windows, macOS, and Linux uses .mar files.

And as I said those are not normal Firefox setup .exe names with the random numbers on end in your image.

If you were to download Firefox 102.15.1esr for Windows from mozilla.org like https://www.mozilla.org/firefox/all/#product-desktop-esr the file name would be Firefox Setup 102.15.1esr.exe for example.