When you look at the actual list of those 4, it's not as hard to understand any more.

It's Firefox, Dillo, Links2 and Netsurf GTK :)

Dillo is something I'd love to daily drive like I did 20 years ago, but it would just fail on most modern websites. But it's what, 2MB in total (binary+libraries)?

Links2 is text terminal oriented. No modern browser can do that natively at all. All competition is even smaller (w3m, lynx). Plus links2 can run in graphics mode, even on a framebuffer, so you can run it without X server at all.

So Fx is the only "general purpose" browser on that list, but is just too big for old hardware.