Opened Firefox and switched to a different tab group by selecting the "Group your tabs" button. After the tabs in that group opened, I again selected the "Group your tabs" button. Some of the thumbnails in the just-opened tab group showed a graphic for the wrong site. I expected each thumbnail to show a graphic for the site represented by the thumbnail. The same behavior occurs for all tab groups. This behavior began immediately after upgrading from Firefox 6.x to 7.0. I restarted Firefox in safe mode and the problem went away. However, disabling all my add-ons did not fix the problem. I found that disabling hardware acceleration fixed the problem. I also fixed the problem by re-enabling HW acceleration and toggling gfx.canvas.azure.enabled to False in about:config. Here's my graphics information: Adapter Description: ATI Radeon HD 5800 Series Vendor ID: 1002 Device ID: 6898 Adapter RAM: 1024 Adapter Drivers: aticfx64 aticfx64 aticfx64 aticfx32 aticfx32 aticfx32 atiumd64 atidxx64 atidxx64 atiumdag atidxx32 atidxx32 atiumdva atiumd6a atitmm64 Driver Version: 8.892.0.0 Driver Date:9-8-2011 Direct2D Enabled: true DirectWrite Enabled: true (6.1.7601.17563) ClearType Parameters: ClearType parameters not found WebGL Renderer: Google Inc. -- ANGLE -- OpenGL ES 2.0 (ANGLE 0.0.0.686) GPU Accelerated Windows: 1/1 Direct3D 10 OS is Win 7 x64 (SP1) CPU is Intel i7 May be a duplicate of bug 659278, but I thought the information about toggling the value of gfx.canvas.azure.enabled to False might be helpful.

It's indeed a duplicate. I will add the information about gfx.canvas.azure.enabled to bug 659278. Thanks!