Just found a new article in the help center: What browsers do we support?
And it contains link to Browserlist. I don't see mobile Firefox here.
Was it by mistake or this browser is not supported?
Firefox for Mobile is not officially supported and never has been on our network. It has always been in a state of "it works but may break" in our list of supported browsers on Meta (which in fact still mentions version 10.0.3 from back in 2012).