User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:59.0) Gecko/20100101 Firefox/59.0 Build ID: 20180323154952 Steps to reproduce: Running Firefox 59.0.2 on Windows 10 Pro 64 bit latest updates I attempted to open up the English original version of the Bitcoin original papers PDF file linked here: https://bitcoin.org/en/bitcoin-paper Actual results: The result was an seemingly endless loop of tab openings of said PDF file. See the video included. Expected results: Only one tab opening of the selected PDF.

I would like to add that this bug has affected every PDF file I have opened recently.

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/