I updated Firefox to the latest version 6.0.1 a couple of days ago. Today, I tried to create a New Category and the screen locks up. I can't do anything on that screen after that happens. No saving, nothing. I cannot even cancel the category creation to try and exit the screen. Nothing works.
I seem to be able to get around in most other areas of the back-end, but I haven't tried everything yet.
Just found something really strange. I turned off RokPad and switched it to No Editor and the problem went away. So it seems that this isn't a problem with MC after all, but with RokPad instead.