I don't think Firefox uses meaningfully more energy due to "optional features", but rather due to simply not optimizing for battery efficiency at the same level that Apple does for Safari.
That type of optimization requires tons of profiling and is less glamorous than implementing new features, so I could see how it's hard to prioritize for Mozilla, especially if optimizations might look very different across OSes.
That type of optimization requires tons of profiling and is less glamorous than implementing new features, so I could see how it's hard to prioritize for Mozilla, especially if optimizations might look very different across OSes.