I just installed JabFox and I have installed JabRef via jabref_5.0.0-1_amd64.deb from https://builds.jabref.org/master/.

However I don't see any additional menu or something like this in Firefox (v70.0 (64-bit)) and when I look into the JabFox preferences, it says

Error: Attempt to postMessage on disconnected port

for the connection status.