Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste pdfjs.enableScripting and pause while the list is filtered

(3) Double-click the preference to switch the value from true to false

I'm not sure how to test that it worked. Maybe Adobe has a safe example on their website?

By the way, it was false up to Firefox 87 and flipped to true starting in Firefox 88.

I only see that pref set to true in Nightly.

• https://searchfox.org/mozilla-central/source/toolkit/components/pdfjs/content/PdfJsDefaultPreferences.jsm
• https://searchfox.org/mozilla-beta/source/toolkit/components/pdfjs/content/PdfJsDefaultPreferences.jsm

• 1702796 - Update pdf.js to version 2.8.320
  • #13116 Enable scripting by default in the viewer (PR 13053 follow-up)

I wonder whether this one overrides that one?

https://searchfox.org/mozilla-release/source/modules/libpref/init/all.js#343

Thanks for the info - It was Firefox 88 release notes where I saw the bit about JAVASCRIPT in PDFs. I wish I could change the title on this issue to say "Javascript in Firefox 88 PDF form viewer" - upgraded my test system to FF90 then upgraded 'production' to FF88. Usually my questions are associated with the test 'nightly' version. I overlooked it when nightly went to version 88.