Is this still true? I just tested and it works for me on LibreWolf, both for google.com and google.com.br.
Honestly, Google doing this as a deliberate anti-Firefox measure seems so wildly stupid and counterproductive on their part that I'd assume it was some failure (serving a slightly different version of the page to Chrome as for other browsers, and the non-Chrome side breaks for some reason) before thinking it was malicious.
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:
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.
LibreWolf is a desktop browser, Mull is a mobile browser. Firefox for Android has the same issue while desktop Firefox does not, so I assume that's where the problem is.