Zotero (standalone) 5 is out and doesn’t work together with the current JabFox AddOn anymore.
I know there is a plan to improve JabFox this way. I still see JabFox as an AddOn in my Firefox 56 but the toolbar-button for JabFox is gone. Currently I workaround it with store all references in Zotero and than export/import them to/into JabRef from there.