I always prefer to download updates myself, to then upload each to VirusTotal.com to see what the results are from the many scanners VT uses, before installing any software obtained from over the Internet. VT always begins the upload to the VT website by checking for a hash and this worked prior to Firefox 91.0.x, but has failed for all three updates since the last version before 91.0, so failing for 91.0, 91.0.1 and 91.0.2. The same thing also happens with Thunderbird 91.0.1 from Thunderbird.net, adding this note in case you can communicate regularly with the TBird team. I'm glad to do the downloads, verifications and installs myself, for if anything ever goes wrong, then I have the downloaded installers to just re-install Ffx. But given that a hash isn't found for the downloaded 91.0.x updates, then I of course wonder what Mozilla is using when performing automatic updates. After all, what we download surely should be what Mozilla uses as well. So, I'll definitely stick to my preferred way of doing updates and definitely hope that this missing hash problem gets corrected very soon. My current browser is still 90.0.2. It definitely is a "bit" buggy, flaky, often quite annoyingly so, but its hash was found and is still found by VT, so ....