You can't check this with code that runs on the server or via JavaScript in the web page.

Note that current Firefox version do not expose the built-in PDF Reader via the navigator.plugin array, so if you can't detect a plugin than maybe assume that the built-in PDF reader is used with Firefox version 19 and later.

• Bug 840439 - Expose PDF.JS as a plugin navigator.plugins when enabled

my issue is: If my FF installed adobe reader already, but in option>application. I still select "Preview In Firefox" So, I can detect the plugin exist or not but I can not check what selected option In my picture, I installed Adobe and Foxit but the selection is Preview in FF and I expected that I can detect the browser is using Preview in FF to show PDF

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".

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.

• http://kb.mozillazine.org/about:config

See also:

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