This KB2670838 is pure evil. It kills Firefox. I hope somebody in European Commision will notice this recent move from Microsoft ;-).
No, actually, it doesn't do that.
My Win 7 Pro x64 system installed KB2670838 on 27 February 2013, the day after it was released. Because I almost never install optional updates, I assume that KB2670838 was released as an "important" update that was automatically installed. I use Firefox
all the time (currently at 21.0) and have had no problems with it. whether caused by KB2670838 or anything else, for that matter.
Plainly, there is an interaction between the files installed by KB2670838 and something else that appears to be present on many Windows 7 systems. But the problem isn't caused by the update
per se.
I have elected to block the installation of IE 10 because the computers for which I'm responsible are used on proprietary websites that often are not updated to work with the latest IE for quite a while. Perhaps the problem is with IE 10 in combination with
KB2670838.
The article that goes along with this update notes several issues, from relatively minor ("If you are a Windows 7 DirectX developer who uses the June 2010 DirectX Software Development Kit (SDK), you will have to update your development environments after you
install this platform update" (a matter of updating 4 dll files) to relatively major (if you have any of a whole slew of laptops that use
a combination of Intel and AMD video cards, you'll get a BSOD with stop code 0x50).