avast-5 update blocks pc for minutes when FF3.6 is open

several times i noted that the pc is blocked for 5 minutes (or more) when avast-5 is doing/intending update and ff3.6 is open. nothing works - and NO data-transmitting via internet shown ???
after 5 minutes (or more) the open windows begin to flicker and its possible to start working normal.
has anybody the same problem ???