Fix the 'scrollbutton' in Tabbar is always not visible in Firefox 32+.

I has tested the in Fx31 and Fx32, that is ok.

This bug is due to:
https://bugzilla.mozilla.org/show_bug.cgi?id=878020

References:
https://hg.mozilla.org/releases/mozilla-release/diff/bbfe5c6ea7c9/toolkit/content/widgets/scrollbox.xml
http://hg.mozilla.org/mozilla-central/raw-file/2255d7d187b2/toolkit/content/widgets/scrollbox.xml