Update April 8, 2015: OS X 10.10.3 shipped today, and as hoped, the final shipping version does not in fact break FontLab Studio 5.1.4 (or 5.1.2; earlier 5.0.x versions are probably also okay but not tested).
We would like to take this opportunity to reiterate our appreciation for our friends and colleagues at Apple: they proactively alerted us to the problem, and then later changed path to avoid triggering it with the actual shipping 10.10.3 update when it came. A class act all around!
As announced April 1,
FontLab Studio 5.1.5 build 5568 (release candidate 1) is available (since replaced by FontLab Studio 5.1.5 build 5611, release candidate 2). Because it is possible that a future Mac OS X build will cause the same problem we previously saw, we will continue testing 5.1.5 and expect to make it available in the future.