I have a 2015 Pro and even if Safari uses less battery, I feel the same thing about Firefox. It is fast and smooth and has less bugs than Safari. However the thing that made my laptop feel cool again is the overwhelming amount of extensions available. There are so many of them and they really improved the ux. Safari has way less and most of them are commercial projects by big techs that collect datas or paid extensions by indie devs that I still can’t 100% trust because they’re closed source. Most of the extensions you find with the search bar on firefox is open source.

Safari was also buggy for me for some of our service web pages like Ambari - for some reason Safari took 1 core to 100% and internet stopped working everywhere. Only solution was to kill the process. With Firefox, everything seems to be working perfectly (albeit Youtube loads slower). I also find the redesign delightful and visually appealing.

For such a dramatic page-loading difference, I wonder if your Mac is using a not very good DNS that Firefox is avoiding because it defaults to DNS over HTTPS.

Good point, but I just checked and DoH has been disabled this whole time. The difference isn't that dramatic, but shaving off around 100ms on every page load really adds up in the end.