Firefox had an OS, and it was rebranded to KaiOS, which is still extant and active today. FirefoxOS/KaiOS, however, is targeted only for feature phones, and th

Firefox had an OS, and it was rebranded to KaiOS, which is still extant and active today. FirefoxOS/KaiOS, however, is targeted only for feature phones, and therefore, resource-constrained systems. I did enjoy one such phone, and it had a fairly decent browser, though I can't say that it had any Firefox-like features that I could identify, because it was so incredibly minimal.