Firefox - Still the only Really Useful Browser.
"...even Firefox, which seems to be sliding further into irrelevancy every day..."
Really? News to me. Firefox is the only browser available that does what I need, and now I can't use another without getting impatient and frustrated very quickly. Its performance has improved a great deal recently, so it now sits quietly in the background when you're not using it without chewing up memory and CPU (although if you've been using it for a long time you may have to disconnect sync, then go directly to your sync account and empty it for a fresh start, to make this happen).
There are many useful extensions, some of which I use on a regular basis, and others which I enable only for specific purposes (like CertPatrol, which has been useful in the wake of FREAK) - if they're not loaded, they don't affect performance of course.
Do browser tests on Peacekeeper (http://peacekeeper.futuremark.com/) and Octane (https://developers.google.com/octane/) in order to get a grip on the realities here.
I occasionally use Chrome and IE, just to remind myself of how inadequate browsers used to be. I've tried Vivaldi, which I assume is an IT high school project. It ran HTML5 video on YouTube impressively, but otherwise makes me wonder, as I do for so many Linux distributions, why the developers are bothering. There must be something better they can do with their time and expertise.