Browsers Power Consumption
This is most likely not the Browser's fault. I guess you're on some badly implemented (bloated) pages that are packed with tons of JavaScript.


Try disabling JavaScript and compare to the full-featured version.
Also AdBlocking or plugins such as uMatrix (Chrome Add On, FireFox Add On) allow you to selectively disable features (frames, js, media) for certain vendors. Maybe you get an idea what may be going on in modern websites :)
Have a look at your Browser's Development tools. Chrome for example has some very nice features hidden behind the Performance tab. It shows you what happens on your page (rendering, parsing, cpu/gpu usage)


But this is quite a broad field you're about to enter here. Maybe have a read on web performance before you start digging into the tips above.