Hmm... I can't reproduce that here, I use Firefox too. Which kind of computer do you use?
A couple of related tips you might find helpful:
-There are hotkeys for using Mastodon entirely through a keyboard (More info: https://fedi.tips/using-mastodon-through-a-keyboard/)
-There is a "slow mode" that stops timelines scrolling, this might be a workaround for whatever bug you're experiencing? (More info: https://fedi.tips/what-are-the-local-federated-home-timelines-how-do-i-stop-them-scrolling-too-quickly/)