Originally this file was shared, using pre-processor statements, between the generic PDF.js library and the *built-in* Firefox PDF Viewer. In the latter case the `getXhr`-option is being used, see https://searchfox.org/mozilla-central/rev/f32d5f3949a3f4f185122142b29f2e3ab776836e/toolkit/components/pdfjs/content/PdfStreamConverter.sys.mjs#612-621