If you go to the Firefox options (tools > options, or click the menu button on the far right of the toolbar, then click "Options"), then search for PDF, then in the list that shows up, find the PDF entry, and in the dropdown to its right select "Preview in Firefox", not "Use Firefox (default)" or "other" or anything like that. There are a collection of relevant bugs here: - bug 1122340 for an obvious "preview this file in Firefox" option in the "open with" dialog (ie the dialog in attachment 9116487 [details]) for compatible filetypes; - bug 1581383 for the fact that there are multiple options displayed in the Options list - bug 215554 for infinite loops of tabs opening if you manage to change the default option to "open with external app" set to Firefox The first seems most relevant here.
Bug 1603749 Comment 13 Edit History
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If you go to the Firefox options (tools > options, or click the menu button on the far right of the toolbar, then click "Options"), then search for PDF, then in the list that shows up, find the PDF entry, and in the dropdown to its right select "Preview in Firefox", not "Use Firefox (default)" or "other" or anything like that -- that should fix things so opening http://www.pdf995.com/samples/pdf.pdf directly opens in Firefox's built-in PDF viewer. There are a collection of relevant bugs here: - bug 1122340 for an obvious "preview this file in Firefox" option in the "open with" dialog (ie the dialog in attachment 9116487 [details]) for compatible filetypes; - bug 1581383 for the fact that there are multiple options displayed in the Options list - bug 215554 for infinite loops of tabs opening if you manage to change the default option to "open with external app" set to Firefox The first seems most relevant here.