I use Iceraven currently. It’s a Firefox fork. Back when I started to use it, It was one of only two browsers (other being Smart Cookie Web Preview) to support extension sideloading on Android.

I don’t think Iceraven is as hardened as Mull, but it is updated on a regular basis. Either case, for most folks, regular Firefox should do the trick. There is also Waterfox, another FF fork on desktop/mobile but it hardly has much to distinguish itself.