User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0 Build ID: 20180605171542 Steps to reproduce: Try and open a PDF file, I've been trying to open them from a google search/already on a website. Actual results: The same tab gets opened many times, still continuing to duplicate even if i manage to beat the speed its multiplying at with ctrl W speed and close the original tab. I can fix it by closing the browser and restoring session, in which the tabs that were opened stay there but don't multiply any more and I can close them. They're just blank tabs with a file path in the URL, the actual PDF never opens. Expected results: The PDF should have 1 instance opened and I should be able to view it.

1. Enter about:support into the location bar. 2. Click the Open Folder button. This opens a file manager window with your Firefox profile folder. 3. Exit Firefox. 4. Back in the file manager window, delete the handlers.json file. If you get a "What should Firefox do with this file?" dialog and you choose to open with Firefox, that will create an infinite loop. That dialog is for external applications only, and you're not supposed to be able to pick Firefox there [1]. To do that safely, use the Open in Browser extension [2]. The only way to view PDFs in Firefox is to set the action to "Preview in Firefox" in about:preferences. However, servers can still trigger the "What should Firefox do with this file?" dialog, usually by sending the Content-Disposition: attachment header. One way to get around that is the Bypass Forced Download extension [3]. Less commonly, they can also fail to correctly identify the file type by sending Content-Type: application/octet-stream. Extensions like Header Editor can override that [4]. [1] Bug 218257 [2] https://addons.mozilla.org/firefox/addon/open-in-browser/ [3] https://addons.mozilla.org/firefox/addon/bypass-forced-download/ [4] https://addons.mozilla.org/firefox/addon/header-editor/