User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:62.0) Gecko/20100101 Firefox/62.0 Build ID: 20180830143136 Steps to reproduce: Recently turned on option: "Do this automatically for files like this from now on." when clicking on a hyperlink which lets you view/download a pdf. For: What should Firefox do with this file? Option is Open with Firefox(default) Actual results: First weird thing is that it doesn't actually automatically open the pdf file with Firefox when I click on a hyperlink. Instead, I have to click OK every time. Main issue is that it kept opening the file in new tabs on a loop, until I close Firefox manually. Expected results: File should have opened just once in a new tab.

see bug 1454365#c2 how to fix this.

See also bug 1288990 Steps to reproduce. If Windows is set to open PDF files using Firefox and you set the Firefox option to open PDF files to "Use Firefox (default)" then this is actually a duplicate of bug 215554.