Since May, we've been running our unittest suite twice for each checkin: once for the current reference counting build + test job, and once for the packaged unittests. Our end goal is to run unittests on our optimized and debug builds. We would stop doing our current reference counting builds, since the debug builds also have reference counting enabled. To get there requires a few intermediate steps:

• Turn off tests for the current build+tests unittest job. We're hoping to do this soon. The tests run on the packaged builds would still be active at this point (first phase of bug 507540).
• Turn on tests for debug builds (bug 372581). There's a bit of work left to do here to make sure that hung processes don't kill the buildbot master with multi-gigabyte log files.
• Turn on tests for optimized builds (bug 486783). This would include nightly and release builds eventually as well, and will give us test coverage on our windows PGO builds.
• Turn off original reference counting builds completely, along with the tests done on these builds (second phase of bug 507540).

At some point we're also going to be changing how we run mochitests, by splitting the tests up according to which directory they're in, and then running each of the subset of tests in parallel on different machines (bug 452861).