Thank you for your hard work on this. Unfortunately, it opens a second instance of Jabref even though I have "remote operation" on and listening to port 6050. The import pop-up window then freezes in this second instance without populating the entry and in the first instance it creates a new untitled library.

I am writing this review mainly to help Linux users get this working properly, since I just managed to do so myself. Will update this review when I use the plugin extensively

Note: this guide assumes you have jabref >= 4.2 and the JabFox extension installed. Also, for all of the files quoted below, you can always create the file by copy-pasting the appropriate text.

Step 1:

Download or create the file org.jabref.jabref.json, which can be found here:
https://github.com/JabRef/jabref/tree/master/buildres/linux/native-messaging-host/firefox

put it in either

/usr/lib/mozilla/native-messaging-hosts/org.jabref.jabref.json
(i.e. for system-wide configuration)

~/.mozilla/native-messaging-hosts/org.jabref.jabref.json
(i.e. user specific configs)

Step 2:
Check if the file /opt/jabref/lib/jabrefHost.py exits. If not, download jabrefHost.py from here:
https://github.com/JabRef/jabref/blob/master/buildres/linux/jabrefHost.py

and put it in /opt/jabref/lib/

Step 2.a:
Mark /opt/jabref/lib/jabrefHost.py as executable, i.e.:
sudo chmod u+x /opt/jabref/lib/jabrefHost.py