You just develop the OS. Nothing wrong with that but I fully expect most phone distributions of FirefoxOS to be rather locked down by carriers and phone manufacturers. If Jolla releases the code shortly after their device comes on the market, I don't really consider it much more closed than such a locked FirefoxOS phone.
More importantly, Jolla might very well have good incentives to open up development in time. Once they release and things work out from a business perspective I think it's not unlikely that, piece by piece, development opens up.
Last but not least, I'm not a big fan of the HTML5 and web fad. Unless the web starts moving to the Affero GPL I pick a GPL native app any time over a web app from a freedom perspective. That is not meant to put down the awesome work Mozilla does for the web, don't get me wrong - I just happen to think the whole direction is dangerous.