Choose your preferred PDF viewer in Firefox - Mozilla Support
The most direct way to change this behavior is through the General Settings menu: open pdf in browser instead of download firefox
To stop Firefox from automatically downloading PDF files and instead open them directly in your browser, you must adjust the browser's application settings. This is a common issue often caused by a previous choice to "Save File" being set as the default. How to Open PDFs in Firefox (Standard Method) Choose your preferred PDF viewer in Firefox -