What does that constitute in your opinion?
Because if it's "go back to XUL", that's:
1. never going to happen,
2. would invalidate A LOT of work that's been done to make Firefox more performant and more secure and reduced technical debt (IIRC XUL leeched into FF internals all over the place, making pretty much any sort of update to the browser into a breaking change for extensions)
Just starting in an end and fix the worst omissions would be a good start.
But, it is amomg other things to make a function available that will remove the top tab bar or at least make it invisible.
Not exactly hard, I do it manually every single time I restart my Firefox Developer edition. (Yes, the chrome (i.e. frame) around the browser window is still written in markup. I go into it with inspector and enter display:none at the right place.)