There are two or three related problems: (1) The plugin check site considers anything but the very highest version number of the Adobe Acrobat plugin to be "outdated" (2) Adobe's version numbering schemes vary between its classic and continuous releases of its DC products and plugins (3) Adobe's downloadable installers do not always include the latest security patches, you have to get those by starting the product (e.g., Adobe Reader DC) and using its Help menu, "Check for Updates" As a result, you can end up in a loop of frustration. My recommendation is to use the internal updater inside your Adobe product. Adobe Acrobat/Reader XI and Adobe Acrobat/Reader DC are still supported with updates and will grab the latest version of the plugin along with other program files when you use the internal updater. To ensure that Adobe's updater can update the plugin, either exit Firefox during the Adobe update or close all PDFs you are viewing in the browser.