Just read the thread and realized no one answered your question!
Firefox was the last major browser to drop Windows XP support. By that time, the last official versions of IE and Chrome for XP were hopelessly outdated, so FF 52.9ESR became the default choice of WinXP users for Web browsing.
Then, Moonchild Productions forked a very recent version of FF, 52.6ESR, for their Basilisk browser, and used its UXP engine for the next version of their Pale Moon browser (28).
MCP disabled Windows XP support in their forks, but @feodor2 and @roytam1 were able to build XP-compatible versions of both of MCP's browsers. With these, XP users were able to continue browsing the modern Web. For several more years, those browsers were indeed the best choice for Windows XP.
Unfortunately, Google is playing "Monopoly" these days, developing new Web "standards" almost daily, that are first supported by new versions of their Chrome browser. Even Micro$oft (no stranger to the game of Monopoly itself) has jumped on the Chrome bandwagon, abandoning both IE and their original Edge browser in favor of a Chromium-based version of Edge. Firefox is the only major browser still using a different engine, but even it looks and feels a lot like a clone of Chrome these days.
So today, the UXP-based browsers are starting to show their age. These days, the browser best able to handle the modern Web for Windows XP users is probably 360EE, an XP-compatible Chromium browser developed in China. But the XP-compatible forks of MCP's browsers are still popular, especially with those still trying to resist assimilation into the Goog
Apologies for the number of USA-based pop-culture references....