FCP 5.1.2 appears to have a “crash on startup” conflict with the latest Avid Quicktime Codecs.
(the latest available ones from August here: https://www.avid.com/onlineSupport/br…ID=404&browse= )
Remove the Avid Codecs and FCP fires up just fine…put them back and its crash city. They played nice under 5.1.1.
If you are having crashing issues with this update, check in Library/Quicktime to see if you have the Avid codecs installed, or any other third-party codecs.