It depends. When a server says that a file is of the application/pdf content type, it should load in the built-in PDF viewer, or in the Adobe plugin. However, if the server says it is a different content type, even if it has a .pdf extension, directing Firefox to open it can lead to an infinite loop of more and more tabs or windows opening. The typical strong medicine for this is to rename the settings file that associates the problem content type with Firefox. This also will delete any past associations created for different content types and revert to the default. Open your current Firefox settings (AKA Firefox profile) folder using either * "3-bar" menu button > "?" button > Troubleshooting Information * Help menu > Troubleshooting Information In the first table on the page, click the "Show Folder" button Leaving that window open, switch back to Firefox and Exit, either: * "3-bar" menu button > "power" button * File menu > Exit Pause while Firefox finishes its cleanup, then rename '''mimeTypes.rdf''' to something like mimeTypes.old Start Firefox back up again and you should get the standard behavior. This time, if Firefox doesn't know how to handle a PDF, please do not redirect it to open in Firefox. (If Firefox knows how to handle it, it won't pop the question in the first place.) If I've misunderstood what happened, could you give a link to the problem download? ''Removed stray text from copy/paste error.''