I have downloaded Adblock. I want to put it on Firefox. When I open the download it says "There is no default application specified to open the document "adblock_plus-1.0.2-fx+sm+tb.xpi". I can either cancel of "choose application". When I choose application, nothing comes up I can do this with. Basically I don't know what to do or how to do it. I could not find the info on Adblock.org. Any suggestions? thank you.