I would like to change the context of the javascript executed in the webkit developer tool/firebug console to execute its code like it is running from inside an iframe on th
Chrome 15 allows you to change the scope of the console. On the bottom of the console, next to the clear console button, there is a menu that says which will give a list of available frames:

Firefox has a similar feature currently in development:

You can also navigate across frames using the command line:
var frame = document.getElementById("frame1").contentWindow;
cd(frame);