I just began to see this problem, but it doesn’t occur with Safari, or Chrome on the same sites. The page appears to load fine on my iPhone X (iOS 12.4; FF19.0), and the page seems to scroll the first time, but sticks once it stops. It won’t scroll up or down. I keep trying to move the screen, and it eventually scrolls again for some unknown reason, but once it stops it sticks again. Here’s one page on which it did it to me: https://www.huffpost.com/entry/7-of-the-best-places-to-buy-glasses-online-for-cheap_n_5af332e9e4b09bb419e45db2