I was given a Firefox OS phone at Mozfest last year. I lost my other phone earlier this year and so used it exclusively for four weeks. I'm sorry to say it's pretty terrible at this stage. I was using v2 of Firefox OS.

The problems are broadly that it was slow, unreliable and poorly designed. It would just become completely unresponsive a couple of times a week, requiring the battery to be removed to restart. There are very few well known apps. Twitter seems to be the only one, and it was so slow for scrolling that I gave up using it. The included apps were just placeholders to install apps, almost all of them only worked online. I wouldn't recommend getting one to anybody. I'm sure you could buy a cheap secondhand Android and have a better experience. It was worse than Android 1.6.

There's such a huge mountain of work for the Mozilla developers to get this to be in anyway competitive to Android. I think Mozilla would be better to concentrate their resources elsewhere.