1) Run the Adobe PDF Reader plugin detector.
2) Run the Firefox PDF Reader detector.
3) When both detections are complete, we show the final results...
4) If the Firefox PDF Reader is the default pdf viewer, then a warning is shown that the browser will use a non-Adobe PDF Reader (by default) to display pdf documents, regardless of whether the Adobe plugin is installed or not.
If the Firefox Reader is not the default viewer, then we just show the normal detection results for the Adobe plugin itself.
Feel free to use the Javascripts shown above, and adapt them to fit your own needs.