This is deeply weird and should be impossible.
On an HP DV6100 laptop running a freshly-updated Lenny, I use Iceweasel 3
and the Flash library from Adobe, also updated today.
I reinstalled Lenny last week. Starting Friday (I didn't use the laptop
over the weekend) and continuing today (Monday), if I open any web page with
a Flash movie on it, the computer freezes. I don't mean that X freezes, I
mean the entire system.
As a test, I ctrl-alt-F1'ed to a VT and ran "sleep 3m; killall xdm". I've
tested that invocation, and it does in fact kill X when the sleep expires.
Then in an xterm I ran "script iceweaselfreeze.txt ; iceweasel &".
Iceweasel loaded. I navigated to YouTube and tried to play a Captain
Disillusion video. The first frame loaded, and then everything froze. The
command in the VT failed to kill X, and I was forced to power-down to reset.
The file "iceweaselfreeze.txt" is empty.