This is if I try to test the workings of a videoDocument, by using an iframe. If the src I use is local, it goes into what looks like streaming. I have a tryserver result of with the test I would expect to pass: https://tbpl.mozilla.org/?tree=Try&rev=2c47a9772f56 This was my test: https://hg.mozilla.org/try/rev/2c47a9772f56 If I switch out the ok() and simpletest calls, to console.log, and run it outside of mochitest, it works as expected. It also does not go into the streaming mode, so I suspect that's what is causing it. I does not fire a seeked event for the mediaFragment that never happens, because when it tires to seek, it just streams instead. It seems.