Well, it was getting stuck on TLS handshake for me on many web pages (I saw it in the bottom left corner of Firefox), that's all I can say.
I just checked, and have updated my Firefox from 60.0.2 to 61.0, and re-enabled HTTP scanning. It looks like it works now. This one page that got stuck before comes right up.
This was quite frustrating.
Can't they build it into the program that if it's holding something up, it lets you know? That would be an improvement.