I'm referring to the one built-into Firefox, not the HTML version. Even comparing the HTML one, this is misleading: the PDF.js compilation step means that there

I'm referring to the one built-into Firefox, not the HTML version. Even comparing the HTML one, this is misleading: the PDF.js compilation step means that there are WebAssembly code written nominally in JavaScript, namely those that handles font rendering and complex graphics (before pushing into the canvas, which by necessity is done in JS).