Mozilla/5.0 (Android 10; Mobile; rv:121.0) Gecko/121.0 Firefox/121.0.

I just did a bunch of testing. The issue is that final version number, "Firefox/121.0". Google returns very different versions of the page based on what browser you claim to be, and if you're on mobile Firefox, it gives you different mobile versions depending on your version:

% wget -O - -nv -U 'Mozilla/5.0 (Android 10; Mobile; rv:62.0) Gecko/121.0 Firefox/41.0' https://www.google.com/ | wc -c
2024-01-08 15:54:29 URL:https://www.google.com/ [1985] -> "-" [1]
    1985
% wget -O - -nv -U 'Mozilla/5.0 (Android 10; Mobile; rv:62.0) Gecko/121.0 Firefox/62.0' https://www.google.com/ | wc -c
2024-01-08 15:54:36 URL:https://www.google.com/ [211455] -> "-" [1]
  211455
% wget -O - -nv -U 'Mozilla/5.0 (Android 10; Mobile; rv:62.0) Gecko/121.0 Firefox/80.0' https://www.google.com/ | wc -c
2024-01-08 15:52:24 URL:https://www.google.com/ [15] -> "-" [1]
      15
% wget -O - -nv -U 'Mozilla/5.0 (Android 10; Mobile; rv:62.0) Gecko/121.0 Firefox/121.0' https://www.google.com/ | wc -c
2024-01-08 15:52:04 URL:https://www.google.com/ [15] -> "-" [1]
      15

If you're an early version of Firefox, it gives you a simple page. If you're a later version of Firefox, it gives you a lot more complete version of the page. If you're claiming to be a specific version of mobile Firefox, but the version you're claiming (edit: oopsie ~~doesn't exist or even really make sense~~ didn't exist when they set this logic up or something), it gets confused and gives you nothing. You could argue that it should default to some sensible mobile version in this case, and they should definitely fix it, but it seems to me like it's clearly not malicious.

Edit: Wait, I am wrong. I didn't realize Firefox's version numbers went up so high. It looks like the cutoff for where the blank pages start coming is at version 65, which is like 2012 era, so not real old at all. I still maintain that it's probably accidental but it looks like it affects basically all modern mobile Firefoxes, yes.