Nobody knows. While they're a similar approach to development, the company, strategy, and goals are different. HP wanted an OS for their smartphones and tablets that would compete with iOS. Firefox is aiming for something more along the lines of an ambitious Linux distribution for the touchscreen era.
Given some basic analysis, I think it's quite safe to say that it'll be a failure.
Android and iOS are already extremely well entrenched, and they're where the money is for developers. There are other OSes, including Blackberry OS and various Windows versions, out there an actual phones already, too. At best, FirefoxOS is starting at 5th or 6th in line, although it's probably further back than that. That's a lot of catching up to do, especially given that it isn't really revolutionary or unique in any beneficial way.
Android and iOS give developers much more flexible environments to work with. To many experienced developers, especially those with a diverse background, being forced to use JavaScript, HTML5 and CSS is just not going to be seen as an advantage. If anything, it should be considered a reason not to target Firefox OS.
Things haven't been very bright for Mozilla lately, either. Firefox's usage is dropping, in favor of Chrome and even IE in some cases. This isn't surprising, given how it has been one mistake after another since Firefox 4. Thunderbird is on the rocks. Firefox for Mobile has been a complete failure. Aside from Bugzilla, which is looking pretty outdated these days, their other projects don't have much of a user base.
When one takes an impartial look at Firefox OS, there's just nothing compelling about it. There's no reason for its existence, and it doesn't really offer anything we couldn't already do. It seems to me to be more of a "we can do that, too!"-style project driven by vanity, rather than any real need. They'd be better off putting the effort toward improving Firefox on the desktop, which seems to be their only remotely viable project.
