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I'm having the same problem, and as described above, it happens on the first launch of Firefox after starting windows. After that if I close Firefox and open it again the Speed Dial page will load without problems.
Windows 7 - 64

Firefox 65.0.1

FVD Speed Dial (Up to date, sorry no idea where to check the version, but Firefox tells me there aren't any updates for my extensions)

It just hangs there with the spinning (loading) animation.