See bug 1148403 comment 36. When in the add-ons manager if the user sees an add-on is disabled for signature problems they may want to try to contact the developer for help.

We think there's a few different places for end users to go already.

(In reply to Andy McKay [:andym] from comment #1) > We think there's a few different places for end users to go already. Yeah: for any problem about a specific add-on: - If the author has gone AWOL there's nothing anyone can do about it - Forward-thinking authors may have left an add-on forum URL and/or a user contact address at AMO; and/or a "home site" URL clickable from the add-on details at about:addons - If everything else fails, a user may leave a "Review" on the AMO add-on page, but only as a last resort: the author will usually be sent an email, but he may or may not react. - Or maybe the XPI in question was a "nightly build", an "experimental version" or similar, intentionally not signed and not meant to be used with release builds of Firefox. But in that case there should have been a caution wherever the addon was downloaded from. (Developers know all this, of course; I'm detailing it here for the poor newbie who might have stumbled on the problem, and found this bug by looking if it was a known problem.)