Flow team had a this Firefox no-JavaScript test that was failing (since fixed).
If you follow the link in the test result
Sauce Labs job URL: http://saucelabs.com/jobs/6496614169424ae8974ae69154f900e4
and view the screencast, you can see that all it shows is an empty browser window. It's not capturing the test state at all. I noticed the same for an earlier build, (change 476 to 473 in URLs).
The test sends an old user agent so that MediaWiki ResourceLoader does not load most JavaScript ResourceLoader modules, so that the test simulates a no-JavaScript browser. Perhaps this makes the Sauce Labs screencast technology not work.
If this is a fact of life, extensions/Flow/tests/browser/features/flow_no_javascript.feature should have a comment "Sauce Labs' screencast of this test doesn't capture anything, see phab:86707."