Sometimes we don't detect problems in Firefox until many weeks or months after code has landed on mozilla-inbound. We do builds for almost every checkin that happens, but end up having to delete them after a month to keep disk usage under control. Unfortunately that means that if a problem is detected while in the Beta cycle or even after release, we're only left with nightly builds for regression hunting, and so the regression window can be up to 24 hours. A lot can change in 24 hours!
Nick's hard work means we are keeping a full release cycle's worth of mozilla-inbound builds archived on S3. We're focused on mozilla-inbound for now, but I imagine we'll be adding other integration branches shortly.
The archive is currently available at http://inbound-archive.pub.build.mozilla.org/pub/mozilla.org/
Please send Nick your thanks, and if you run into any issues, please file bugs blocking bug 765258