On some boxes the Flash plugin will always crash your web browser. 
Maybe this helps you too, if you have spurious crashs with Firefox/Iceweasel.
Here are tips to avoid this:
1.  Edit your 
/etc/X11/xorg.conf as root:
 Disable Composite 
Comment out the following line:
----8<------8<------8<------8<---
Section "Extensions"
...
#       Option  "Composite"     "Enable"
...
EndSection
----8<------8<------8<------8<---
 
 Set Color depth to 24 
Change the following line:
----8<------8<------8<------8<---
Section "Screen"
...
   DefaultDepth   24
...
EndSection
----8<------8<------8<------8<---
 
 
Now you have to restart your X Server. 
2.  Another Fix
Execute the following line before starting Firefox/Iceweasel
export XLIB_SKIP_ARGB_VISUALS=1
 
Enjoy Kanotix!
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