Firefox 4.0 on Vista the browser will run fine for 30 min to 3-4 hours then begin freezing for increasingly long periods on each mouse click or attempt to move the scroll bar. Eventually it locks up. Problem is most noticeable after returning to FF after i'ts been the inactive window for a period of time. Sometimes there is a message that a Java script or Flash plugin has stopped working, other times nothing. Same results regardless of cancelling/continuing script, leaving or reloading plugin. Typically, first click on the FF window is met with the (Not Responding) warning, then a long delay; window becomes active, another click or scroll bar move initiates another long delay, etc. It may recover for a while, but then after seemingly random periods starts freezing for long periods. Memory and network activity appear normal during this time. Have run FF with all themes and extensions disabled, and updated all plugins, but same problem. No crash report - as the FF simply locks up until I close it with TaskManager. Any suggestions?