capybara, webdriver not launching on xvfb display

Hey Guys,

I'm trying to run cucumber (capybara) in a xvfb, I've set it up
correctly, and can manually launch firefox using "DISPLAY=:99
firefox".
I'm 100% this works, because I've set up a vncserver bound to that
xvfb, and I can see firefox comes up..

But if I DISPLAY=:99 cucumber, firefox doesn't bind to that display
and times out after 60 seconds. Any ideas on what could be causing
this and how I can resolve?

I've even tried to launch xterm into the xvfb and run cucumber in
there, with no luck. DISPLAY=:99 rake cucumber:all doesn't seem to
work either. I'm thinking this has something to do with the way
webdriver launches a firefox instance. I can't seem attach it!

Any help would be appreciated.

Many thanks,
Mark