The Chinese-made ZTE Open runs applications written in the web-based HTML5 language rather than developed for a proprietary operating system.
A ZTE spokesman said the Open, already on sale in Spain, Colombia and Venezuela, is aimed at first-time smartphone users.
In Britain and the United States the Open, featuring a 3.5-inch display screen and running a 1GHz processor, will be not be locked to a specific mobile wireless provider, the company said.
The decision to sell the phone on eBay "is an admission by ZTE that most carriers around the world are not yet ready to sell Firefox OS devices," on analyst said.
"Being different isn't enough" in a marketplace already crowded by Google, Apple and Windows, Jan Dawson, chief telecoms analyst at British consultancy firm Ovum, told the BBC.
"If you have to explain to people why Firefox provides a better user experience, then it's not particularly compelling," he said.