The Mozilla Foundation (in partnership with GeeksPhone) has just revealed that the first "Developer Preview" phones will be shipping with the new, Open Source, "Firefox OS" very, very shortly - possibly as early as just a few weeks from now.

A few things worth noting:
1) These phones are being clearly labelled as "Developer Preview" models. The purpose of these phones is to get a fully functional Firefox OS-based device into the hands of software developers as quickly as possible. More developers = more new Firefox OS-specific software.
2) Just because these are "Developer" phones doesn’t mean they are real cell phones. Nokia released some Linux-powered "Developer" phones a few years back...and those are still in use today as primary cell phones.
3) No pricing has been announced, though Geeksphone does say that they will be available "at a price you could never have imagined." I’ll be honest: I can imagine some pretty doggone cheap prices...but it’s still a good sign that we might see the first batch of Firefox OS phones for less than half a grand (USD).
There are two models:
Both models top out at 3G speeds, which is a bit on the crummy side. Especially considering this is Firefox OS we’re talking about.
Curious about Firefox OS? Here it is in a nutshell:
Take the Linux kernel. Put Gecko (the runtime that powers the Firefox desktop web-browser itself) on top. Give HTML/JS/CSS (aka “HTML5“ in this context) applications the ability to access phone hardware features - such as making a phone call and GPS.
That... is it. The idea is to create a platform that is easy for web developers to build applications for - and to keep consistent (and, theoretically, Open Source) code-bases between web and phone.

It’s definitely an interesting idea, though not really a new idea. In fact, it seems like a mild variation of Apple’s original iPhone strategy. The original iOS was all about building web applications for mobile devices as well. The crew over at Mozilla seem to be taking a little better approach to it (in my opinion), but I’m a tad bit skeptical as to how the Nerdy public will react to it.
Especially in light of some of the other Linux-powered phone Operating Systems that are available, such as Sailfish and Ubuntu Phone.
But, hey, competition is good, right?