Tor Browser update from 6.5a2 to 6.5a3 on OS X is broken

mrphs reported that his update to 6.5a3 was broken on OS X. He got a

LSOpenURLsWithRole() failed with error -10810 for the file 
/Applications/TorBrowser.app.

error. I tested that on my old 10.6 system and got

posix_spawn("/path/to/TorBrowser.app/Contents/MacOS/firefox", ...):
Permission denied

Not sure whether both are related.

Updating from 6.0.4 to 6.0.5 is working fine and starting a vanilla 6.5a3 is working as well.