I've done 2 complete OS 10.6 sytem re-installs this week and tried various fixes. The problem is confined to files from my Firestore DR-100 and 5 years of archived footage, always edited in FCP, and that worked a couple of weeks back but no longer works. The "Pacifist" fix worked for me to open the files but caused a number of other issues with Compressor and a number of other oddities like deleting App icons.
I did a fresh re-install of OS10.6 (Snow Leopard) on a wiped drive and left iTunes at version 9. something i.e. no updates, Quicktime 7.6 I installed from the Snow Leopard install disc. Everything worked. Then I did the Combined OS 10.6.8 upgrade through the Software Update (but refused all the iTunes and Quicktime 7.7 updates) back to square one Quicktimes recorded on Firestore new and old won't open. Quicktime can't be just uninstalled and an older version re-installed as it becomes part of the whole OS, meaning the only way to go back is a complete system re-install.
Fortunately I have a redundant system drive I can boot from with OS 10.5 but converting 5 years of archived footage is ridiculous. I also have all the original footage archived on tape from the JVC HD111e. VLC will also open the files old or new without any problems on the "updated" OS 10.6.8 system.
Recording new media as m2t on the Firestore and then re-wrapping as .mov using Clipwrap 2 works. Go figure, it's just a re-wrap not a conversion.
Using a trial version of the Digital Rebellion Pro Maintenance tools says the files which I couldn't open are fixed, but they still won't open. This is definitely an Apple update problem I've eliminated all other causes and provoked the problem by simply doing an Apple update.
I've had a very bad year with Apple equipment and applications and need to consider whether their product is any longer viable for professional use. I was considering purchasing the Adobe Production Suite but I don't think this would necessarily solve the issue of the Firestore recordings, but I might try a trial version of Premier.