It seems users are currently being updated to firefox without any warning. Afterwards when searching for a browser firefox starts and tells the user that a new profile was created. I was able to help by restoring the profile from ~/.purism/purebrowser but that was only because i was around.
Regular users are left completely stranded having to go to the safe and get passwords again, go out to web searches for their bookmarks, etc.. Debian performed the icedove -> thunderbird migration automatically on first thunderbird startup. A similar migration technique would help here too: https://wiki.debian.org/Thunderbird (it's even a bit simpler since there's no huge mail folder to migrate in $HOME).