It's currently impossible to install the mozilla irccloud instance because the manifest only lists marketplace.mozilla.com as an installation source. The simplest fix is to remove this line from the manifest: "installs_allowed_from": ["https://marketplace.firefox.com"],
I have made the change, please let me know if it works or not.
Looks like it's fine: stas_> thanks guys, it works now! thanks Dan!
Oops, looks like I still have to learn Bugzilla's experimental UI...