While an end user can switch around among viewers by changing their Firefox options, unfortunately there currently is no way for websites to detect which viewer is in use and assist visitors in a personalized manner. I think for now you may need to recommend that Firefox users use the above article for assistance if more complex PDFs do not render as expected.

You can set the pdfjs.disabled pref to true on the about:config page to disable the build-in PDF viewer.

You can check the value of the plugin.disable_full_page_plugin_for_types pref on the about:config page and remove the application/pdf part if present or reset the pref to the default via the right-click context menu.

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

See also:

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