Occasionally when Firefox starts up, this screen shows up.
Another symptom of this problem is in Explorer. Open Explorer and attempt to select a file by clicking on the filename. All files above the file you are trying to select are also automatically highlighted. Only your selected file should be chosen
Most likely, your keyboard SHIFT key is sticky. Lift gently on the each SHIFT key and attempt to launch Firefox. If it starts normally, simply clean your keyboard, focusing on the SHIFT key you were lifting when it began behaving normally again.
- BG & BB