I have upgraded from ubuntu10.04 to 12.04 and installed lxde. The panel is now customized according to my convenience. The problem is there was a default checked option for "Flash when there is any window requiring attention" in Task bar(window list) and now when I run firefox, at times the concerned bar in the panel window list starts flashing but it is not clear exactly why at that moment firefox requires my attention. Normally this happens after I download some files (pdf files) and keep using firefox. The only solution I have found is to quit firefox and even after that I receive this message

"Sorry, ubuntu12.04 has experienced an internal error. If you notice further problems, try restarting the computer."

As usual I report the detailed message to ubuntu as suggested and finally restart the computer. So there are two problems, first, exactly why firefox needs my attention at times and second, why immediately after this, ubuntu experiences an internal error. Please suggest what I should do.