Hi!
I've been using TopScroll in Chrome for a long time, and it's become a habit to try and scroll pages that way: it just feels right.
Now, I'm thinking of Firefox, at least for a while. Apparently, you could publish most Chromium extensions to Firefox (or, at least, Firefox Quantum). I've tried to use TopScroll in Quantum in debug mode, to no avail: errors appear in relation to the manifest and script contents, which is above my paygrade.
So, I have to ask: could you possibly port the extenstion to Firefox? From the sound of it, there are few obstacles to doing so, since the API supported are mostly the same between the browser engines.