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I too have tiny test app - and that does not have the problem either.
did you see my earlier post about the change from v.4.0.2a to v4.0.7 ?
I've done some more looking into this, I'm not sure why it doesn't crash on other browsers. The references repeat over 5000 times and then the debugger crashes, the merge code has no extra checking for nested references.
The fix is to use Ext.apply, not merge.
Twitter - @evantrimboli