fixes #1431 Slider from markup functional test fails for FF

FF still hasn't fixed its bug with getComputedStyle('display') and ```<style scoped>```, so I changed this test.
For what-is-the-base-issue see https://gist.github.com/jconniff/edf8f728589e4b458d16

I checked it on the following:
FF 26
Chrome
Safari
WinXP IE6
WinXP IE7
WinXP IE8
Win7 IE9
Win8 IE10
iOS6 Safari
Android 2.3.4

Needs Android 4.* testing