I don't know why it doesn't work in Firefox 30, but it works fine in the current Aurora (Firefox 31.0a2 2014-06-09). Assuming you want to stay on the beta channel, all you have to do is wait a few days at most, and then the problem should solve itself once Firefox 31 becomes the beta version.

• https://wiki.mozilla.org/Features/Release_Tracking
• http://www.mozilla.org/firefox/beta/all/

Still works for me if I toggle this pref on the about:config page.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes