Backbone.js requires underscore.js to be in the global page context
I was loading underscore.js then applying the .noConflict(). When Backbone (0.5.1) was loading, it couldn't find Underscore (window._). This manifested as a Firefox console error "f is undefined".
Once Backbone has loaded, you can safely clean up both libraries without disrupting either:
var b, u;
b = window['Backbone'].noConflict();
u = window['_'].noConflict();