We've just introduced a change to Mozilla's Mochitest harness to improve test run times, as per Bug 367393. This involved the removal of unnecessary MochiKit usage. We found that we were including a minified version of MochiKit, packed.js, in all our tests and within the harness, but we would only use a small portion of this enormous suite. That added an extra load of about 5 minutes per debug test run, so we removed MochiKit from our harness and added replacement functionality to SimpleTest. Note that the SimpleTest.js file in MXR may not yet be updated, so pull the latest mozilla-central code to see latest changes!