True, sans all the Firefox branding and such they could do that without Mozilla's consent. With how Verizon went out of their way to ensure their Galaxy S3 was the only one that prevented users for easily unlocking the bootloader and enabling system writing, I don't foresee a positive future with any open OS on Verizon. I'm probably switching to T-Mobile when my contract is up next year. I enjoy the unlimited LTE dataspeed on Verizon I still have, but I'm not interested in keeping it while paying full price for a phone I don't really want.
But they wouldn't be able to call it FirefoxOS. The source may be open but the brand & trademark certainly aren't.
Yeah, I think you're correct. They'd have to remove all Firefox branding/advertising if they went against Mozilla's wishes.
Perhaps something could be added into the license agreement to prevent such usage?