I've been very happy with FF power usage on macOS. It's still a web browser and with all of the nonsense that comes with the "modern" web, it's still a relatively heavy application, but a tab suspending extension will go a long way toward fixing that, as will an ad blocker. https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta... is what converted me from Chrome to Firefox. Coupled with a couple bits of config (https://github.com/cweagans/dotfiles/tree/master/Library/App...), it's a really nice experience.
Also, you may be interested in https://www.ohtipi.com/ for SMS security codes.