Selenium can't launch firefox without sudo in Rails cucumber testing

Question

I have a weird bug when trying to run selenium with cucumber testing in Rails 3. (This is under an RVM running 1.8.7)

My colleague's dev environment works just fine but when I try and run "AUTOFEATURE=true autotest" or "rake features" I get the following error:

unable to start Firefox cleanly, args: [] (Selenium::WebDriver::Error::WebDriverError)

(you can also see selenium trying and failing to launch firefox in the dock.)

Now I've tried a bunch of things to fix this.

1. I compared dev environments with my colleague's mac and made sure we had the same versions of everything
2. I wiped firefox of my computer and reinstalled it. I even tried chmod it to 777 (that's right I went all unix on its ass)
3. I set up a brand new RVM and bundle installed from scratch

I somehow managed to stumble upon the fact that if I ran the test suite with Sudo it would work. Still, this is rather weird and I was wondering if anyone had encountered this before or had a suggestion of something I could try...