Downloaded a Firefox Addon on addons.mozilla.COM. Ignored Untrusted Certificate. Should I worry?

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http://superuser.com – This site: https://en-us.add-ons.mozilla.com has an untrusted certificate. (At least with my browser: Iceweasel 24.2.0) I find that highly suspicious. But I do not know enough about such certificates to be sure. Can someone enlighten me? (HowTos)