I have never had any performance or memory related issues with firefox, on a modest xps 13 9343 with 4gb ram. In fact, with regular usage, firefox rarely exceeds 2gb usage, and cpu usage is never an issue. Though I have tweaked my firefox quite a bit, I don't think the core issue is with the browser technology. The technology is fantastic and computers are fast. Rather, I see it is a combination of two things: poor web browsing habits (tabs open, websites visited) and the ever-increasing badness of "modern" websites.