why is it that there are a million chromium-based browsers but none (afaik) based on firefox? have they not built a reusable core for it or is there some other reason

@clarity
Librewolf, Waterfox, Icecat — all of these are based on FF, not sure if they are still maintained. But there is little point in it as they are basically Firefox with different defaults.
And it used to have a reusable core, but I don't think it's still maintained. Sadly, they have abandoned it and webapp/site-specific-browser mode too 😩