Firefox is driven by Mozilla, which has long since lost the plot and shows no sign of regaining it. If Firefox were forkable we'd have seen one by now, but even Brendan Eich himself chose Chrome as the base for his browser.

I would love to be proven wrong, but my prediction for ten years out is that Mozilla will finally have gone under and Firefox won't be maintainable any more.

(Writing this from Firefox. I'll hold out as long as it's possible to hold out.)