What would you do different if you were rewriting Firefox from scratch?

I would make it embeddable. The goal would be to become the Lua of web engines. As it turns out, having an embeddable web engine would be a massive market position right now.

Stop all JS, CSS, and any other processing for any tabs that are not currently active where the site has not been placed on an allow list allowing it to continue to run in a background tab.

Also stop JS, CSS, etc., for any active tabs where the Firefox window does not have current focus unless on the allow list, above. Firefox currently assumes any active tab, even for an iconified window, shaded window, window on non-current virtual desktop, or otherwise not currently visible window, should be allowed to burn through CPU/battery while doing nothing of value. Using window focus would allow for stopping all non-active/non-visible windows.

This would improve battery life, and leave CPU capacity for other tasks that aren't just wasting the CPU.