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 
https://bugzilla.mozilla.org/show_bug.cgi?id=507540).
* Turn on tests for debug builds 
(https://bugzilla.mozilla.org/show_bug.cgi?id=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 
(https://bugzilla.mozilla.org/show_bug.cgi?id=486783).  This would 
include nightly and eventually release builds as well, and will give us 
unittest coverage of our windows PGO builds.
* Turn off original reference counting builds completely, along with the 
tests done on these builds (second phase of 
https://bugzilla.mozilla.org/show_bug.cgi?id=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 
(https://bugzilla.mozilla.org/show_bug.cgi?id=452861).
If you have any comments or questions, please follow up with me 
privately or on the lists!
Cheers,
Chris