I followed all three suggestions in the support article I found. The "last resort" option was to completely remove and reinstall Firefox. I deleted all traces of the application, then ran the installer. I got the Firefox "first-run" page. All good. I checked the preferences, and under "Applications" --> PDF, the selected option is "Preview in Firefox". However, the first time that I click on a PDF, I still get the same old "Save/Open" dialog, wanting me to download the file or open with Adobe Reader. I would really prefer to just have PDF files open directly within the browser! Why will the built-in viewer still not handle PDF files, even after completely re-installing Firefox?