Asking some friends of mine about why they don't use Firefox and that's because other browsers has resources cap options that could limit memory and cpu usage. Firefox could add this and then simply enable a preconfigured option for battery safe mode...
Maybe another firefox version could be hard to maintain since there is already some versions Mozilla is already maintaining. Maybe an option to reduce characteristics or be slower but more power saver. Brave does this pretty well, so it's an interest...
