In Options > Applications, you can set the action for PDF files to "Preview in Firefox." I can't confirm, but you might need to download the PDF file in order to make changes to it or fill it out.

You will have to use an external PDF Viewer if you need to fill forms.

Current Firefox versions have enabled a built-in PDF Viewer that doesn't have all features that other PDF readers like the Adobe Reader have or may not function properly otherwise.

You can change the action for Portable Document Format (PDF) from Preview in Firefox to use another application like the Adobe Reader or set to Always Ask in "Firefox > Options/Preferences > Applications".

See also:

• https://support.mozilla.org/kb/disable-built-pdf-viewer-and-use-another-viewer

If the Applications panel of the Options dialog shows the option of "Use Adobe Acrobat (in Firefox)" you also could try that if you don't want to launch all your PDFs in the stand-alone Adobe Reader or Adobe Acrobat application. (cor-el's post has the link to the relevant support article)