http://sdrv.ms/14bw06m Here is the video of this problem. You can see the scrolling bar can not be rendering correctly. And this problem only happened on Gmail.com I try to reset the Firefox, and it works fine on Gmail, but once I sync my addons and plugins back, this problem is back. Then I try disable all the plugins and addons, this problem is still exists. If I disable the hardware acceleration in Firefox, the scrolling bar is Okay I try to run Firefox on Intel Graphic Card and Nvidia Graphic Card, this problem happened on both two graphic cards.