I don't know why the Adobe Reader plugin would suddenly stop working; is it up-to-date? If you haven't updated Adobe Reader in a few months, now would be a good time (there was a security update not that long ago). In Adobe Reader, choose Check for updates on the Help menu. You may want to close Firefox during the installation. Another thing you can try is bypassing the plugin and sending the PDF to Adobe Reader. To set this, open Adobe Reader (or Acrobat), choose Preferences from the Edit menu, click Internet and then clear out the checkbox for View PDF in Browser. Firefox now should give you the Open/Save dialog when you click a link to a PDF. If that's too inconvenient, try setting it back and see whether refreshing the settings helps. Any luck?