I also observe the same problem with Firefox on Windows and Linux. I have an impression that their developers don't have any priority on the battery drain, instead they just measure speed, and don't want to investigate too much on reducing actual work done, even when, from the user perspective, we're "doing nothing" (e.g. just keeping the browser open on some pages). I know there are many sites today that have tracking codes etc that "always run" but it seems that it's an excuse not to investigate what the browser "always does" in all the scenarios when it should just do as little as possible. Also, different kind of "movement" on the pages (videos, animated gifs, sounds) all take more battery than the competition.
My measurements are that on some specific pages, even when I "do nothing" Firefox uses up to twice as much CPU. Luckily, it's not always twice as much battery drain, because even idle uses power. But it's definitely visible and measurable: from 3 hours of other browsers you'd probably have only 2 of Firefox.
My measurements are that on some specific pages, even when I "do nothing" Firefox uses up to twice as much CPU. Luckily, it's not always twice as much battery drain, because even idle uses power. But it's definitely visible and measurable: from 3 hours of other browsers you'd probably have only 2 of Firefox.