Current Firefox versions have enabled a built-in PDF Viewer that doesn't have all features that othe

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 the Adobe Reader or set to Always Ask in "Firefox > Options/Preferences > Applications".

https://support.mozilla.org/kb/change-firefox-behavior-when-open-file

You can set the pdfjs.disabled pref to true on the about:config page to disable the build-in PDF viewer. You can also check the value of the plugin.disable_full_page_plugin_for_types pref and remove the application/pdf part if present.

See also:

https://support.mozilla.org/kb/view-pdf-files-firefox-without-downloading-them