Firefox ESR, since it what comes with Debian's desktop. But the browser alone doesn't do anything if your browsing and privacy habits are poor.

Definitely use some of the plugins mentioned here and use something like a Pi-Hole device to filter out unwanted junk (analytic trackers, ads, etc) at the network level. Can go all out and serve normal CDN junk locally too, things like fonts and javascript.

Just sort of depends on what you're wanting to do.

But to answer your question: Firefox for normal use (with some plugins and network stuff on my end).