How to go back to use Adobe Acrobat Reader as the PDF previewer in Firefox browser:

How to go back to use Adobe Acrobat Reader as the PDF previewer in Firefox browser:

1. open a new tab
2. about:config
3. Search for pdfjs.disabled, and then change its value true
4. Search for plugin.disable_full_page_plugin_for_types, and then delete application/pdf from its string value.
5. Done!
6 (optional) You can also fix other extensions by deleting their associate strings from the plugin.disable_full_page_plugin_for_types string value. For example: application/vnd.fdf, application/vnd.adobe.xdp+xml, application/vnd.adobe.xfdf

You may need also to Enable the Acrobat Reader plugin via add-ons of your Firefox browser.





Credits:
https://support.mozilla.org/en-US/questions/950939

See also:
http://helpx.adobe.com/acrobat/using/display-pdf-browser-acrobat-xi.html#Mozilla_Firefox