The issue appears to have resolved itself with a fresh install. I could not replicate the behavior in the nightly, and when I reinstalled the main release it did not manifest again either. Sorry to not be a better help, I'm trying to wrap my head around it...
All good. It's frustrating when problems come and go like this, but not much we can do about it.
If this starts happening again and you get any more data on it, please email me: bzbarsky at mit dot edu.
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.