Firefox 3.5 uses the adobe reader in Firefox to open a PDF file. And if you want to open a PDF file downloaded using Firefox with the default Adobe reader installed in your machine, then you can specify this using the Firefox options. Similarly if you have problems opening a PDF in Firefox then you can check the file association using this Options.
The application association settings can be accessed using Tools -> Options -> Application tab.
Choose the Adobe Acrobat Document as the Content Type and specify required application value in the action drop down. If you want to use the default Adobe Reader 8.1, then select Use Adobe Reader 8.1 (default) value is the Action drop down.
This also allows users to choose any other application for the content type using the Use other option. You can select the required application by locating through the Browse dialog box.