Firefox loads PDF.js using a resource URL (resource://) and those are sourced from a JAR file (omni.ja) so you would have to find that file in your local installation and patch the archive with your customized version.

I’m approaching 30 but I started using Merlin a few years ago and have evolved into a casual bird nerd since.

Sounds like Hyrum’s Law.

From Wikipedia:

There has long been a popular belief that this was a deliberate feature of the design, to assemble aluminium aircraft without overtightening the fasteners. There is no good evidence for this suggestion, and the property is not mentioned in the original patents.