The new Mozilla Firefox OS Developer edition smartphones went on sale as expected, but sold out of stock just a couple of hours after being released on Tuesday, April 23.

A smartphone OEM startup put the first Firefox OS developer phones on sale, offering the Keon for $119 and the Peak for $194. The two devices mark the first hardware with Firefox OS on board.

Both the Keon and its more powerful sibling, Peak, are essentially preview devices designed to allow developers to test out their possibilities with the new Linux-based open-source mobile operating system. Mozilla's new Firefox OS is set to launch to the general public in June in five initial countries, with six more to follow by the end of the year. The Keon and Peak are developers' opportunity to test things out before that general public release.

Smartphone OEM startup Geeksphone was the first to put the Firefox OS developer phones on sale, and both devices have been marked "Out of stock" since a couple of hours after going on sale. The Geeksphone handsets are quite modest in terms of hardware specs, but Firefox OS will target a mid-range market.

The Keon smartphone, which sells for $119 unlocked, packs a 1GHz Qualcomm Snapdragon  processor, a 3.5-inch HVGA screen, 3G and EDGE cellular connections, and a modest 3-megapixel camera. It has 512MB of RAM and just 4GB of internal storage, but features a MicroSD card slot for additional memory.

The $194 Peak handset, meanwhile, sports a 4.3-inch qHD IPS display, a 1.2GHz dual-core Snapdragon S4 processor, an 8-megapixel rear-facing camera and a 2-megapixel front-facing shooter. The rest of the specs are similar to the Keon's.

While both devices are currently out of stock, TheNextWeb reported on Monday, April 22 that Geeksphone plans to fulfill orders at a rate of roughly 5,000 per day. TechCrunch also contacted the company and learned that Geeksphone is working hard to get more phones back in stock and hopes to get new units on sale this week. A company representative told the publication that the phones were in tremendous demand.

While these early results of two developer handsets selling out cannot accurately predict the potential success of Mozilla's Firefox OS, the great early demand is a good sign.