Mozilla's Firefox OS is still in its early fledgling stages, and Spanish manufacturer Geeksphone has probably been its biggest proponent. But getting people to commit to spending the cash for a phone that runs the unproven OS with primarily only webapps available has proven a challenge, and thus Geeksphone's going the dual-booting route with the new Revolution.
On sale today, the Geeksphone Revolution costs €222 (before tax) ($305 USD) and is capable of booting into either Android or Firefox OS. So you can try out the open source Mozilla OS without making a full commitment. Maybe you'll like it, maybe you'll just spend your time on Android.
Now, €222 doesn't get you a highly impressive phone, though what you get for the price isn't bad. The Revolution spots a 4.7-inch qHD (540x960) iPS display with an 8 megapixel camera on the back and 1.3 megapixels on the front. Chipset-wise there's a dual-core Intel Atom Z2560 clocked at 1.6GHz, 1GB of RAM, and 4GB of storage (expandable via MicroSD). There's a standard compliment of Wi-Fi b/g/n radios and Bluetooth 3.0, though the HSDPA radio is a bit of a bummer. But, again, at that price it's not a deal-breaker.
The Revolution is actually an Android phone first, booting to Android (version number is unspecified, though by the press images it's 4.x of some sort, and it appears to be running Google apps) on first activation. Though you can switch to Firefox OS whenever you like. The question is, though, would you?