VIMEO support answered :
You need to have flash installed to watch Vimeo videos in Firefox on OS X because they don't support h.264 playback.
Ah hum yes ok I just reinstalled my Mac a few days ago thats why...
But could they make their message more precise no?
Instead of a silly joke and
This video cannot be played with the current setup
I would have prefer
This video needs flash player installed to be played
Agree VIMEO team?
Thx.