The device was made available through the e-commerce site last Friday (16 August). By Monday, ZTE's current stock had been cleared out by consumers and developers eager to get their hands on the value smartphone, which is the first Firefox OS device to be made available in the UK.
Those who missed out can now sign up to the ZTE shop's newsletter to be notified when new stock arrives.
The ZTE Open comes unlocked so any SIM card can be plugged in. The specs are pretty basic so the main attraction is the low price and new OS; it has a 3.5 inch touchscreen, a 3.2-megapixel camera and 256MB of RAM plus a 4GB micro SD card included.
Despite ZTE's highly successful experiment with direct to customer sales, no carrier in the UK or US has yet picked up the budget smartphone. It is however available through Telefonica in Spain, Venezuela and Columbia.
The other smartphone which currently runs on Firefox OS, the Alcatel One Touch Fire, is available in Poland through Deutsche Telekom's T-Mobile brand.
Firefox has said the firm's new OS smartphones are "the first devices powered completely by Web technologies" and will all come with the new Firefox Marketplace built into the OS.
This integrated system allows the user to search for applications and media without launching an app store. The user can then choose between using an app just once or installing it.