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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)

https://imgur.com/NxH28IN

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