Sure, I'd be more than happy to try breaking this down, I'm going to try a nightly build now as other users have suggested and see how it feels
If you're using a nightly, you should be able to use https://developer.mozilla.org/en-US/docs/Mozilla/Performance... to profile it, because nightlies have useful symbols. Might be worth capturing a profile at the point when the spinner stops and the page is finally shown.
If this starts happening again and you get any more data on it, please email me: bzbarsky at mit dot edu.