If you don't have it please install gdb then run firefox in debug mode with 'firefox -g'. Once you get the crash please post here the output of 'bt full". 
Also please write the url of the site.

I recomplie swfdec  link to gst-ffmpeg but
sorry for that I use firefox3 without debug symbols  and I don't know how to use gdb
so maybe what I can do is  just that post my system config
gentoo x86_64 firefox 3 beta 5 ,gcc 4.2.3  (Gentoo 4.2.3 p1.0)
the USE flag of firefox3  is "dbus startup-notification xulrunner "
swfdec and mozilla - plugins is from git 
configure of swfdec   mad disable   ffmpeg disable    gst-ffmpeg enable   vivified  disable
run firefox from terminal just gives out 
Loading stream: http://stat.youku.com/player/addPlayerDynamicReport
Loading stream: http://static.atm.youku.com/0805/0505/S28/120%20360.swf?url=http%3A%2F%2Fvid.atm.youku.com%2Fhtmlclick%3Fp%3D74%26ca%3D1499%26ie%3D1548%26k%3D%25E5%2590%258C%25E4%25BA%25BA%257CMAD%257C%26u%3Dhttp://news.gyyx.cn/counter.ashx?position_id=79!url!class_id=3!url!page_id=2!url!source_id=5!url!game_id=1
/usr/lib64/xulrunner-1.9/run-mozilla.sh: line 131: 29298 segment fault               "$prog" ${1+"$@"}


and the url is  http://v.youku.com/v_show/id_ch00XMjYwODMzMDA=.html

I am pretty sure this is a problem with your setup. The link you posted works fine  here - using Swfdec on Firefox 3.

Feel free to reopen this bug if you figured out it's a problem in Swfdec.

> ------- Comment  #3 From Benjamin Otte  2008-05-08 15:14:13 PST   -------  
>>I am pretty sure this is a problem with your setup. The link you posted works
>>fine  here - using Swfdec on Firefox 3.

>>Feel free to reopen this bug if you figured out it's a problem in Swfdec.

one more question
I don't know   if  that  your  linux  box  is also  x86_64
I mean  some bugs just  happen    on    x86_64 .
if  your  linux box   is x86_64  too ,that  means   this is  a problem with my setup

One of our core developers - Pekka to be exact - develops Swfdec using x86-64, so I am pretty sure it works as well on 64bit as it does on 32bit.

Also, a lot of the users of Swfdec run on 64bit systems, as that's the system where Adobe's player doesn't have the best history. ;)

seems that it is caused by unstable nspr or nss lib
after install new version of nspr nss everything seems all right