I am not sure whats going on, but I did notice that you had a fairly old version of firefox. Apparently you have the version 28, the new one is 34. I would suggest updating to the new version if possible, they may have fixed that bug. Have a great day! -JonJake_Cole said

I don't know what happened but suddenly all thumbnails in tabgroups started appearing black every time after restarting Firefox. The strange thing is that right after restarting Firefox and open the tabgroups window for the part of the second the thumbnails appear with their old cached version and then all turn black. I reinstalled firefox and run CCleaner but it didn't help.

The problem was caused by one addon, which was working fine, but suddenly started making problems. I uninstalled it and found a similar addon.