This won't work as expected, I don't think?
Unless you installed the developer edition, which uses a separate profile/path, both of your installs will use the ~/.mozilla directory, so configs and profile of your new install will interfere with the ESR version.
(and why did you symlink /opt/firefox/firefox to /usr/bin/firefox? The desktop file already takes care of recognition)
While it's not a feature of lemmy, you can if you absolutely need to, without being subscribed to a kbin instance.
See for yourself: Go to here to see this very thread on kbin, scroll down and click on the "favourites" tab, it will show you precisely who voted how on your post and when. For comments, see hitagi's reply.