Firefox is big enough that there might be enough traction to create a proper fork, if Firefox ever goes down. Though, technically the same is also true for Chrome, since the system behind it is also open source. It's just a lot of work. There's also still Webkit around. That one is often used for web-tech apps for desktop & of course Apple, so i wonder if it's feasible to make a real browser out of that. Midori comes to mind, idk if they still use it.