I've been running Debian Testing (Lenny) for a while with Gnome and Iceweasel however only recently I've been using it regularly.
So I reached a point where I wanted to check a TV schedule and the TV station's website needed Flash. Latter on I've also found that YouTube required a Flash
To fix this and get myself Iceweasel working with Flash websites:
# apt-get install flashplayer-mozilla
Just fire up Firefox and happy YouTube browsing ;-)