I'm surprised to see this, because for all its flaws, Twitter is one of the very few apps that don't lose scroll position for me on either the web or the mobile app.
There is an extremely annoying bug though where sometimes a random tweet will just disappear from the timeline. You'll get a brief glimpse and it suddenly disappears. And you may or may not come across it later. As though it jumped further down the line.
Anyone else notice that? What in the world might be happening there? This has been an issue for at least a couple years now.
Twitter does keep your scroll position when you go from a "top-level" view ("For you", "Following", ...) to a Tweet thread, then come back.
Edit: Nevermind, it happens to me as well, I just never noticed.
I am mildly curious about why only a subset of Firefox users are experiencing this. Do you mind sharing your Firefox version and OS?