Why am I getting this message: "Firefox can't be opened because it is from an unidentified developer."

Firefox icon suddely vanished from the dock and the app won't open.  The message box says, "Firefox" can't be opened because it is from an unidentified developer. Your security preferences allow installation of only apps from the Mac App Store and identified developers."  I have the 'Mac app store and identified developers' box checked in Security/Privacy under Preferences. I updated Firefox 1/17/13.  Now what do I do?

iMac (27-inch Late 2009), OS X Mountain Lion (10.8.2)

Is heer...

Firefox was working until this occurred? From where did you get Firefox? Btw, the latest Firefox was updated to Firefox 19 just a few days ago.

http://www.mozilla.org/en-US/firefox/new/

I certainly would have thought Mozilla is participating in Mountain Lion Gatekeeper code signing.