Hard pop-up dialog boxes and bubbles are so last-millennium. Smooth slide-out notifier panes are all the rage nowadays. If your GUI application does not animate something, it sucks. So much so that Firefox also implements sliding animation to notify users when their downloads are complete.
Not too big a deal, does not suck up too much processor attention for most programs. Until you start consider the amount of network bandwidth suckage it causes just to transfer that animation sequence over the wire. Yep, used over long distances to Remotely desktop another computer, sliding animations are pure pain that make me want to murder the developers' families.
Note to all software developers including myself: the age of remote computing is upon us. Not everybody who uses your application are having the "luxury" of having the window imagery rendered directly from memory to video card. Whatever graphical bells and whistles you design into your pretty program, please include options to turn them all off. We need to realise how expensive it is to replay the animation when there is the Internet (network) inbetween.
Firefox (v3.0) does not offer a direct UI element to control this feature, but thanks to others who peruse the about:config page, there is an option to disable it
I thank thee for not letting Mr Hyde burst out of me.