Found this with google
What I did in Firefox was go to Tools-->Options-->Downloads-->Plugins and disabled the pdf plugin. Once I did that and clicked on a link to a pdf document, Firefox asked me what program I would like to use. I selected the acrobat reader executable and told it to do that typically.
Firefox is now using Acrobat reader to open pdf's and seems to be quite fast. Not only that, but if I go to Tools-->Options-->Downloads then I can now see the pdf filetype in the filetypes window.
Hope that helps!