Firefox uses PDF.js[1] to render pdf. This is a standalone project to render pdf in the browser using html and js. If I remember correctly, you can even render pdf in chrome using this.

[1]https://mozilla.github.io/pdf.js/

It's good, however it's not possible to use the remote debugging protocol for printing to PDF yet.

I just used Chrome headless to render several thousand pdfs last week and I saw no issues in version 73

Did you have a reason to not use PDFium directly?

Likely part of a web scrapping project that may require navigation or other browser things to get to. Or they are trying to avoid detection with a more real browser (you have to change some stuff first).

Can you recommend any resources for getting started with pdfium?

Sorry I'm in that position myself. A couple of years ago I compiled it without too much difficulty, and the included example I tried out rendered a page to a .png. Doing that for that for thousands of PDFs would just take a loop (or find -exec) if you have the files locally.