Already happened years ago, if you want the old Firefox with XUL extension support, full themes and zero tracking as it was before they started copying Chrome i

Already happened years ago, if you want the old Firefox with XUL extension support, full themes and zero tracking as it was before they started copying Chrome in 2011, give Pale Moon a try. It's an independent fork, the last independent browser left, with its own rendering engine Goanna that is a fork of Firefox's Gecko.