Hmm, could you check two places: '''(A) Advanced Configuration Editor''' (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return to load that internal page. Click the button accepting the risk. (2) In the search box in the page, type or paste '''pdfjs.disabled''' and pause while the list is filtered (3) If the preference is bolded set to true, double-click it to restore the default value of false ''Does that work?'' More info on about:config: [[Configuration Editor for Firefox]]. '''(B) Active Policies List''' In a new tab, type or paste '''about:policies''' in the address bar and press Enter/Return to load that internal page. Are any active policies related to PDFs listed here, or just the default phrase: "The Enterprise Policies service is inactive"?