I guess it depends on setting "privacy.resistFingerprinting" (and maybe some others in that section).
Of course, I base my assumptions only further to that forum post, the thread on Firedragon and a very quick Internet search. I have no idea what else could possibly be behind it...
Edit: I can confirm:
- On my work (Windows) Firefox privacy.resistFingerprinting was True and Whatsapp Web showed my messages as UTC. Setting it off, the time was changed to my local time.
- On my Firedragon privacy.resistFingerprinting was True and Whatsapp Web showed my messages as UTC. Setting it off, Whatsapp Web didn't let me in, for having an unsupported browser
