In the README zenhub is mentioned for issue tracking. But I think zenhub closed the doors. Clicking the link provided in the README points to a side without a tls certificate. Also, i can’t find anything in the addon store of mozilla for zenhub.
Thank you for the report. I see the link on our docs page is indeed broken. We still use Zenhub. You can download the extension here. I’ll fix the link soon.