Activation objects (the objects that provide local scope for a function invocation) should not have a __proto__ property. They don't in Mozilla (which originated the __proto__ extension) and it allows various optimizations of property lookup if this is not supported.
Here's a simple test: javascript:alert((function() { __proto__.testVariable = 'found'; return window.testVariable; })()) This should show 'found' and does in Firefox 1.5 (but not in some older versions - presumably that means they decided to change this too from the way it was).
This is also fixed by the changes for 6249. *** This bug has been marked as a duplicate of 6249 ***