I just upgraded from NC 14.0.6 to 15.0.5. (Went very well, WELL DONE, devs!)
I sometimes use an old Firefox version 845.6.0-esr) and this version does not display any contacts at all with the new nc-version; it displays the (one) group that i have with its correct name but empty.
The error does not occur with alter firefox-versions (60-esr and 65-devel)
(The other apps (Files, Acitvity, Gallery, Mail, Tasks, Claendar, Notes, GpxPod, Polls, Tasks work fine.)
Thanks a lot 
Iām not sure I get your issue though.
Could you maybe post some screenshots? Or better detail what is not working?
The page is blank? Are you using latest contacts 3.0.3?
Sorry for the delay - i only use that computer on weekdays.
I use the latest stable version of contact (3.0.3). When i go to contacts the page is empty except for the (empty) group name, the navigation elements in the blue area at the top, the area-dividers and the settings link on the lower left side (cut out of the screenshot).
i do not consider this a serious bug since the browser version is (kinda) outdated but thought you devs might want to know. i can supply additional info if needed.
regards, pd
Hey! Sorry as well for the delay!
Please attach some console logs here as well