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ZTE Open Firefox, the first Firefox OS phone for consumers, makes European debut in Spain.
The handset comes at a price of €69.
According to reports, the phones are set to compete with Google's Android, but only at the lower end of the smartphone market.
Android holds three-quarters of the share in shipments in the smartphone market, but analysts expect Firefox OS to capture 1 percent of the share of global smartphone shipments in 2013.
According to ZTE chief executive Cheng Lixin, the phones could reach the US in 2013 as well.