Now said contributor works a bit more on the project and adds some great new functionality, but floorp don't agree it fits their plans. So the contributor decides to make their own fork called ceilingp and build from that. Nope, they don't have the license to do so. They can take the mpl parts. They can take their own parts (they didn't sign an exclusive release of their code). They can add their own new code. They can't use the rest of the floorp code though.

So floorp gets the benefits but no one else can build off it without permission (save for private use without releasing it and potentially having others do the same).