svking said

I musthave that when I need do download documents

Adobe hasn't released any PDF plugin for Android till now. So direct reading them on browser isn't possible may be. So you have to Save pages as PDF then open them with adobe reader.

When you download a PDF, Firefox should hand the question of how to open it off to the Android OS. If you are having a problem using Adobe when you access the download (either from Firefox's Downloads list, or the system Downloads folder, or the notifications on the top bar), you may need to clear the defaults from the application that is opening the PDF. This should give you back your choice the next time.