Seems running Firefox as root in a regular user's session is not supported.
linux Firefox -v report:Running Firefox as root in a regular user's session is not supported. ($XAUTHORITY is /home/user/.Xauthority which is owned by user)
Any comments/ideas?
You saw it right.
In the discussion Cannot run Firefox as root Daniel Veditz in his comment clearly mentioned:
There is no recommended way to run Firefox as root, and because of our sandboxing techniques it's quite broken even if we hadn't explicitly disabled it through start-up checks. As a "bug" this is not something we're going to fix.
They further added,
"Run Firefox as root" doesn't seem to be your ultimate intention though, more like "How do I manage a shared Firefox installation so it gets updates?" That sounds like a support question rather than a bug. I don't actually know that answer--I don't use a shared machine or multiple accounts so I just install locally--but you can try our support site https://support.mozilla.org/ or our forums https://www.mozilla.org/en-US/about/forums/ or maybe even StackOverflow once more people run into this issue.
This issue was marked with status as RESOLVED WONTFIX