Patch your firefox...

Firefox has long had a built-in PDF viewer, allowing users to view PDF files in the browser without having to install a third-party application. In addition to the other weird things PDF files can contain, one of them is JavaScript. Putatively offered as a way to create self-validating forms, this scripting capability has been abused over the decades in just about every way you can imagine. Firefox's built-in viewer, although it has apparently had the ability to execute embedded JS for some time, never turned that feature on, making it a safe(r) way to open PDFs... Until now. The newly released Firefox version 88 has flipped that switch, and will now blithely execute JavaScript embedded in PDFs. Firefox's main preferences dialog offers no control for turning this "feature" off.

To turn off JavaScript execution in PDFs: Enter about:config in the address bar; click "I'll be careful." In the search box near the top, enter pdfjs.enableScripting. Change the setting to False. Close the page.

 

That's a hell of a whoops on FF's end.