"'Firefox' is an application downloaded from the Internet. #

Are you sure you want to open it?" kept appearing every time Firefox was launched. The quarantine bit was apparently stuck:

   $ xattr -l /Applications/Firefox.app
   ...
   com.apple.quarantine: 0000;51...|com.apple.Safari

Clearing it did the trick:

   $ xattr -d com.apple.quarantine /Applications/Firefox.app

UPDATE: Recently started receiving a similar error when opening a certain app:

You are opening the application "foo" for the first time. Are you sure you want to open this application?

Resolved using kopischke's advice:

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -all local,system,user

/mac | Feb 11, 2013

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