Firefox Developer Edition, Chrome, and occasionally Edge.<p>Web development in Firefox, because I like the basic dev tools better. Performance testing in Chrome because I like its dev tools' network tab and profiler better.<p>Primary social media and store/etc accounts in Firefox. Alternate "junk mail" social media accounts in Chrome. General browsing in Chrome because it's faster than Firefox (less lag on JS-heavy pages), and to minimize tracking. For instance, if I do product comparison on Amazon logged into my account and decide not to buy it, I'll keep seeing it recommended.<p>Also recently started doing some general browsing on Edge, now that there's extension support and can sideload uBlock, to evaluate its performance, memory, and battery claims. So far it seems to be a bit more memory-efficient than Chrome, but also has a bit laggier UI and some annoying idiosyncrasies. Haven't used it on the go enough to get a feel for battery savings.
I use Safari mostly. Firefox sometimes, and I'll use Chrome when I need to something that has Flash. Safari has always been fast enough for me, I like the Firefox plugins/greasemonkey, and really, I don't have much of a need for Chrome because I tend to stay out of the Googleverse.