Hi there, another user with similar issues reported the following fix:

"I was recently having this issue on my laptop with Ubuntu 18.04, with this extension and bitwarden. after a little digging it seems it was a bug with gnome-shell and password fields. after updating packages and rebooting the problem is gone. sorry I don't have the links handy at the moment but try updating, close firefox and reboot."

https://github.com/xbrowsersync/app/issues/63