On November 24th we landed some changes we think are a big improvement to how we've been doing nightly builds.
We've started doing nightly builds on the same revision across platforms, and where possible, on revisions that we already know compile. In addition, all of the nightly builds will share the same buildid.
We pick the revision to build by looking at the past 24 hours of builds, and finding the latest one that built successfully on all platforms. If there is no such revision, we'll build the latest revision on the branch and hope for the best. We also do some extra checking to make sure we don't build a revision that's older than the previous nightly.
Prior to this, we would trigger the nightly builds at the same time every day, but which revision and buildid each platform got was completely dependent on what time the builds actually started. There were many instances of nightly builds having different revisions between the various platforms.
These changes are a big deal because they mean that we have a much better chance of getting a working nightly build every day (or at least one that compiles!), and all the platforms will be based on the same revision, regardless of when they are run. No more guessing if today's Windows build includes the same fixes as the Mac build!
If you're interested in the implementation, which includes some pretty fun buildbot hacking, the details are in bug 570814. Background discussion as to how to choose the revision to use can be found in this thread.