ooh that was what I was gonna suggest - pop up blocker. Can you disable it for mumsnet?
Go into tools, click on pop up blocker and then pop up blocker settings and then add *.mumsnet.com to list of allowed sites. IF have windows that is. IF have google toolbar click on popups ok button when here and it should allow mumsnet without completely turning off popup blocker.
IF its not that don't know what it could be.