I've spent the past couple of weeks using the ZTE Open, which runs the fledgling Firefox OS, as my main phone. The phone came with version 1.0.1 installed, so not long after I got it I built an Ubuntu VM and flashed my phone to run 1.3. The improvements were noticeable immediately; the screen was far zippier, the keyboard was better laid out, and I was able to use push notifications. The ZTE Open really is an entry level phone, and I've read plenty of negative reviews w/r/t it's specs, but for £60 I think it's pretty good. Don't get me wrong, it can't handle multi-tasking very well, and sometimes apps take a little too long to open, but for what it is, it's good.
The underlying concept of the Firefox OS is to leverage web apps, and I've got quite a few bookmarked on my home screen for apps I use regularly, like Wordpress, Twitter, and the BBC. There are plenty of other web apps available, and Grooveshark in particular highlights the Firefox OS ideology that you shouldn't have to write an iOS app, an Android app, a Firefox OS app… You should just have one web app that will run anywhere. Grooveshark runs a user-generated content (UGC) website that allows users to upload sound recordings and other users to stream those recordings. Grooveshark is a contentious piece of software, and concerns about copyrights led Google, Apple and Facebook to remove Grooveshark's applications from Google Play, the App Store (iOS) and Facebook platform respectively. In December 2010 the site's interface was rewritten for HTML5, meaning it was accessible through most devices web browsers, therefore abrogating any requirements necessary for the app to be available via any app portal for mobile devices. It's simple just to search for Grooveshark and to pin the site to your homepage and access the site directly, without even having to go to the Mozilla Marketplace.
Speaking of the Marketplace, I downloaded Loqui. Loqui Instant Messenger is a mobile chat app for Firefox OS that allows you to have all your chat accounts in just one FirefoxOS app. This includes WhatsApp, and even get notifications when someone sends me a message when the phone is on standby, which I don't get on my Lumia 800!
I think Mozilla might be onto something with the Firefox OS, and the reaction seems positive: Several of their hardware manufacturer partners are looking to release more devices and Mozilla will demo new features at MWC later this month (like WheresMyFox, a cloud service to provide users a method for users to locate, track and purge devices remotely.) I'm certainly going to upgrade to a better device once they are available.