WTF ... Mozilla had always running JavaScript inside PDFs disabled by default.

But now with FF 88 this option is ENABLED by default. Which means, if a PDF file contains JS it will run without any user interaction. What can possibly go wrong?

To disable this:

about:config
pdfjs.enableScripting --> false

# FF 78.10 ESR doesn't include this option and still blocks JS in PDFs by default. Just tested.