Making fixtures 'visible' so that UI tests work on Firefox.

I'm using js-fixtures with karma+mocha to do UI testing and I've found that, in Firefox, jQuery will always report elements in the fixture as *not* visible (even if they normally would be). Visibility works as expected in Chrome. If you care to reproduce, I'm using Firefox 25.0.1 and Chromium 30.0.1599.114-0ubuntu0.12.10.2.