I happened to be surfing to meta.discourse on my mobile phone which runs Firefox for Android.
The hamburger menu button is present but pressing it appears to do nothing (I had it open the menu once when playing with the “Request Desktop Site” option).
Browsing using the “classic” version of Firefox on my PC does not appear to have any problems.
EDIT: Hang on,
it seems like it might work when I’m not logged in, I found this out when trying to reproduce it on the Atom forums where my cookie must have expired. I’m going to do some extra investigations. EDIT 2: See below.