Except they limited it with recent Firefox Mobile "updates" or "redesigns". AFAIK the last fully-featured FF Mobile on Android is 68.12.0. I have tried the most recent FF Mobile couple of weeks ago and there were only a small subset of add-ons supported and still no about:config.
yeah, I wonder when the Facebook container will make it to mobile - it's one of their own after all so there should be no safety issue.