That looks like a nasty one, thanks for finding it.

I'm the new webmaster of the nombril.com website.
It contains many flash movies (unfortunatly, they have lost the *.flas), all with sound. I've tested swfdec with all of them. It works well, except for the three ones with sound + href links: I have no sound, firefox crash on one of them and doesn't follow the link on another one.
Here is the files:
www.nombril.com/menu.swf -  no sound, crash
www.nombril.com/illus_accueil_droite.swf   -    no sound, dosn't crash
www.nombril.com/jardin_anim2.swf   -    no sound, doesn't crash but doesn't follow the link either.

Testing this with 0.5.5:

- www.nombril.com/menu.swf: works fine with sound

- www.nombril.com/illus_accueil_droite.swf: no sound and no warnings wrt sound

- www.nombril.com/jardin_anim2.swf: get sound but does not follow the link

So we have an improvement, anybody with x86-64 can confirm these?

Exactly the same results on x86_64 with the current version from git.  No crashes.

I haven't closed it yet, because I wanted to play a while with it to be sure the segfault doesn't happen anymore.
The mouse input rewrite for 0.5.5 should have taken care of it though, so I wouldn't be surprised if it's gone. 

But as you seem to have played enough on it, I'm closing it and if anyone manages to get the SEGV again, please reopen.