Firefox was working fine until I installed Microsoft's .net framework 4. After that, erratic behaviour occured. New tabs would not keep focus. Autocomplete would not work. Random duplicate Firefox windows would open. Pasting words or phrases into forms would result in gobbledgook. Removal of .net framework instantly restored normal operation! So I don't need a solution but am just reporting this in case it helps anyone else who see erratic Firefox behaviour! By the way, I have not seen this problem with earlier version of .net framework so it seems to be number 4 only that causes the problem.