Well to install IE8 Beta 2 was the worst mistake I made..... it required an update to be installed (KB937287) which completely screwed my system over. Interesting point Firefox became the instable point, as long as FF 3 isn't running my system runs like it used to... if FF3 is active and I start another app or game, Firefox just vanishes and/or the app/game crashes.
Sometimes FF3 crashes with C++ Runtime errors (and I have uninstalled all C++ Runtimes :o ), sometimes it crashes with an invisible error box or the Mozilla crash handler......

EDIT: By accident I found out what was wrong..... for some reason my Swap File was disabled (I bet it was the work of that update...) now that my Swap File is running again nothing crashes anymore^^