A PDF that opens in Firefox may need a desktop application for a particular task. First decide whether this is an exception for one document or a preference for future downloads. A one-file choice avoids changing a familiar workflow for everyone using that browser profile.

Open one downloaded file elsewhere

In Firefox’s Downloads panel, find the specific PDF and open its context menu. Mozilla provides an option to open it in the system’s default PDF viewer. Alternatively, reveal its containing folder and use the operating system’s Open with choice for that file.

Check the filename before selecting it, especially when the download list contains several revisions. If the document needs a particular supported application, confirm that application is installed and approved for the work. A button offering another viewer does not establish that the viewer supports the document’s special features.

Make a continuing preference explicit

For future PDF handling, open Firefox Settings and find the Applications or Files and applications section; its location varies by Firefox version. Select Portable Document Format (PDF) and inspect its Action. Open in Firefox uses the built-in viewer. Selecting a listed application changes Firefox’s PDF handling, while Always ask provides a choice when a PDF is downloaded.

Mozilla notes that choosing another default viewer or Always ask also saves the opened PDFs to Downloads. Account for those local copies if the files are confidential. Record the old action before changing the preference.

Use a harmless sample to compare three entry points: clicking its web download link, opening the downloaded entry in Firefox, and opening the file from its folder. Record which application appears for each. The operating system’s file association is a separate setting, so the results need not all match.

Keep the preference that fits the actual task, or restore the recorded Firefox action. Describe the failing entry point when requesting help; “PDFs open in the wrong app” alone leaves the relevant setting unclear.

Source: Mozilla Support.