I tried these phones running Firefox OS at Mozcamp, the annual gathering of folks who build the Firefox browsers and several open source apps and tools. This OS can run on humbler hardware resulting in more affordable smartphones.
The phones will be released early next year - starting with Brazil and then other markets. The phones are going to be much cheaper than existing smartphones. The other advantage is that the apps are built using Web development tools, speeding up availability of apps.
The challenge always is phone makers support the new OS.