The phone specification is similar to the original ZTE Blade  also known as the Orange San Francisco. This originally went on sale nearly 2.5 years ago with Android 2.1 and by today's standards is considered low end.
If Firefox OS provides as good or better user experience than Android 2.1 on such modest hardware, Mozilla have every chance to carve out a market in low end smartphones.
No information on pricing, sadly, but given that a similar phone (with a slightly better CPU and an IPS screen; there is no information about the screen technology used in the ZTE Open but I'd except it to be TN) running Android can be bought from a wholesaler for around $90  for a single unit I expect it to cost no more than that when it shows up for sale online.
The iPhone moved on from 320x480 back in 2010. Three year old tech in a new phone today is a bit odd. Half the resolution of VGA makes it a bit useless for browsing the web. And the browser is kinda the point of a smartphone.
Most of the give-away phones (free on any contract or about $100 without) here in the US have better than VGA resolution, twice the resolution of this bottom-end screen.
>Now, you know what IS inadequate? 240x320 phones. They are genuinely useless for web browsing.
As an owner of a Nokia E5 phone with a 320x240 screen I beg to differ. From personal experience I have come to a conclusion that 320 by 240 phones are only useless if you are using a desktop-like (or even an iOS Safari-like) browser. Opera Mini for the E5 tries to reformat pages to fit the main text content a single column and uses Symbian's own fonts to fit more text on the screen, which makes browsing the web surprising adequate. I wouldn't call it a great experience but it's far from useless. It is particularly noticeable when you contrast the results with using Android's Browser or even the Android version of Opera Mini at 320x240, especially since you don't have to touch the tiny screen.