Webpages jump "up" every so often while scrolling down

This occurs reliable for me right now on Firefox Mobile on https://aphyr.com/posts/358-a-history-of-leather-at-pride-1965-1995 While scrolling down the webpage, occasionally, about half a second after I lift my finger from the page, the viewport "jumps" up 1-2 paragraphs. This can happen more than once after I've stopped scrolling, or not at all. It appears to also sometimes happen while scrolling, but I can't reliably reproduce that part. I've uploaded a video of this to https://i.imgur.com/guNho5G.mp4. It's a bit blurry, but the key point is there: I scroll down the page, slowly, and then right before the video loops, the screen "jumps up" once. This also happens on https://www.egscomics.com/ occasionally, but on that webpage, it's only when I'm zoomed in on the text at the bottom, and scrolling slowly. Much less reliable to reproduce than the blog post on aphyr.com.