If you ask me, Mozilla is already over-committed with Firefox OS... So much so that they are looking for ways to make more money to hire more programmers (read: sponsored tiles). Google pays them 313 million / year to be their default search engine. I wonder if they're spending that all on Firefox OS, which may not be generating any revenue for them. If creating a desktop operating system also doesn't generate any revenue, it would be just another drain on their resources.
It looks like they are the only ones with the resources to do it, so long as Google doesn't threaten their main source of income. Ubuntu doesn't look like it will fly