Does it change to use the Adobe Acrobat plugin (which would not be helpful since Firefox 52-53 don't run that plugin any more), or use your external Adobe application (either Adobe Reader or Adobe Acrobat)? One possible reason for settings confusion is that the file which stores these application handling preferences can become corrupted. Since it has a somewhat inscrutable XML-based format, we generally recommend letting Firefox rebuild it from scratch. And this is how... Open your current Firefox settings (AKA Firefox profile) folder using either * "3-bar" menu button > "?" button > Troubleshooting Information * (menu bar) Help > Troubleshooting Information * type or paste about:support in the address bar and press Enter/Return In the first table on the page, click the "Open Folder" / "Show in Finder" / "Open Directory" button. This should launch a new window listing various files and folders. Leaving that window open, switch back to Firefox and Exit, either: * "3-bar" menu button > "power" button * (menu bar) File > Exit / Quit Pause while Firefox finishes its cleanup, then rename '''mimeTypes.rdf''' to mimeTypes.old. Start Firefox back up again. PDFs should be assigned to "Preview in Firefox" by default on the Options page.