Firefox and chrome use a half-dozen or so threads parsing and compiling JS code so it loads quickly. Firefox's Quantum work adds a lot of threads for rendering as well. In addition, service workers, web workers, and Wasm all spawn additional processes which serve the same purpose if you aren't sharing memory directly.

That said, the Phoronix Selenium tests show AMD ahead by quite a bit. (source)[https://www.phoronix.com/scan.php?page=article&item=ryzen-37...]