Firefox released a mobile OS (Firefox OS) in 2013. The aim was to encourage app developers use open technologies (HTML, CSS, JavaScript, open web APIs). Development of Firefox OS stopped in 2015.
At the time I thought that Firefox was wasting resources on development on Firefox OS. But now I think I was wrong. Firefox could see the promise of a PWA (Progressive Web Apps) mobile future, but the tech was clumsy and immature. Perhaps Firefox OS was too early for the technology?