To achieve what might be the same goal, I simply open a new
terminal window, 'ssh -X otherusername@localhost' (having
ssh authorized_keys and sshd_config all set up to allow it),
and run the browser or other apps from there. It has been
for me, and I hope it's secure though I don't know all the
possible downsides within X security-land.
For one java IDE I have to do 'ssh -Y otherusername@localhost'
instead, before launching it, since apparently it needs things
that -X doesn't allow, and I haven't learned enough yet about
X security to be more specific in what is allowed.
One downside is that the first term window above can't launch
new windows after a while, but that's easy to work around.
There was more discussion on similar things at the thread
(sorry I don't have a link handy) from march 2015, subject
"running multiple simultaneous X sessions as different users".