I have the same problem I finally came to the conclusion it was Firefox after checking other web browsers they work fine I have windows 7 and I can't find a solution either please help thanks

You may have security software that blocks access to specific files as soon as one instance has been started and thus prevents starting another Firefox instance for a different Windows account.

I guess that was a possibility I gave up on version 8 I uninstalled it, googled version 7 and installed that and now it runs fine when I switch user to user Thanks for the help I'll take it into consideration if I update again