[It is possible for 2 or more PDF readers to be in the same browser. And one of those will be the browser's default reader. When the Firefox reader is the default, then the Adobe plugin usually will not be used - even if the Adobe plugin is installed & enabled.]
The following sequence of events occurs when the detector shown above runs:
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.