Alas, Firefox doesn’t allow changing any keybindings without modifying the source code. The bugzilla ticket about this is already 2 decades old.

Incidentally, anyone know which Firefox flag to set to disable running JS in PDFs entirely?

It’s pdfjs.enableScripting in about:config. Note that Firefox is more strict with PDF scripts to begin with, and the linked Linux PDF only works in Chromium-based browsers.