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. Firefox's main preferences dialog offers no control for turning this 'feature' off.

Your pdf files might still appeare as Firefox html documents and. The newly released Firefox version 88 has flipped that switch, and will now blithely execute JavaScript embedded in PDFs. Go to Firefox Options>Applications and changed all Actions for the pdf files to Adobe Reader. 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. 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. In addition to the other weird things PDF files can contain, one of them is JavaScript. To allow the extension to parse local PDF files, you need to enable 'Access Local Files' in the extension manager.

As long as Firefox file type associations exists, you can read text files within the browser window: When clicking on other file types, such as Microsoft Office, Open Office documents, PDF files, etc., if the extension is loaded in Firefox, you will be able to browse the file with in the browser window. The extension provides the same experience as you open PDF files in the Firefox browser.

Ewhac writes: 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. While browsing files systems is convenient, opening files is also possible.

Opening pdf files in firefox