Setting MONO_IOMAP=all environment variable for nunit tests

Hello,

I am trying to set the MONO_IOMAP=all to get selenium webdriver tests running with firefox on the mac. If I start Xamarin Studio from the terminal ( /Applications/Xamarin\ Studio.app/Contents/MacOS/XamarinStudio & ) after setting the environment variable in a shell it works. However if I only add the environment variable in the Options > Run > General settings it has no effect.

So my question is. How do I set environment variables so that they are applied to nunit tests running in Xamarin Studio? ( I tried adding it in code as well but that did not work either :-( )