Apple recently announced changes to how OS X applications must be packaged and signed in order for them to function correctly on OS X 10.9.5 and 10.10. The tl;dr version of this is "only mach-O binaries may live in .app/Contents/MacOS, and signing must be done on 10.9 or later". Without any changes, future versions of Firefox will cease to function out-of-the-box on OS X 10.9.5 and 10.10. We do not have a release date for either of these OS X versions yet.
Move all non-mach-O files out of .app/Contents/MacOS. Most of these will move to .app/Contents/Resources, but files that could legitimately change at runtime (eg: everything in defaults/) will move to .app/MozResources (which can be modified without breaking the signature): https://bugzilla.mozilla.org/showdependencytree.cgi?id=1046906&hide_resolved=1. This work is in progress, but no patches are ready yet.
Add new features to the client side update code to allow partner repacks to continue to work. (https://bugzilla.mozilla.org/show_bug.cgi?id=1048921)
Create and use 10.9 signing servers for these new-style apps. We still need to use our existing 10.6 signing servers for any builds without these changes. (https://bugzilla.mozilla.org/show_bug.cgi?id=1046749 and https://bugzilla.mozilla.org/show_bug.cgi?id=1049595)
Update signing server code to support new v2 signatures.
We are intending to ship the required changes with Gecko 34, which ships on November 25th, 2014. The changes required are very invasive, and we don't feel that they can be safely backported to any earlier version quickly enough without major risk of regressions. We are still looking at whether or not we'll backport to ESR 31. To this end, we've asked that Apple whitelist Firefox and Thunderbird versions that will not have the necessary changes in them. We're still working with them to confirm whether or not this can happen.
This has been cross posted a few places - please send all follow-ups to the mozilla.dev.platform newsgroup.