User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:54.0) Gecko/20100101 Firefox/54.0 Build ID: 20170225110158 Steps to reproduce: Using Firefox 54 (Nightly) on Linux: — I clicked on a link to PDF file attached with a Facebook message. — Facebook servers handle this by offering the browser to download the PDF, not by hosting it in a way that it could be previewed in the browser. Actual results: — So the download dialogue “What should Nighly do with this file? Open with/Save File” opened. — Wanting to see it directly in a Firefox tab, I chose “Open with Firefox”. — An infinite number of empty tabs opened, each opening successively another one. Expected results: Opening the PDF (as it was now downloaded in my temp folder instead of on a Facebook server). Alternatively if not possible, opening no more than one tab and display an error explaining the situation.

That's the case when Firefox calls itself recursively to open a file: https://support.mozilla.org/t5/Fix-slowness-crashing-error/Firefox-repeatedly-opens-empty-tabs-or-windows-after-you-click/ta-p/2008 Did you associate PDF files to Firefox in the OS?

No. Only in the “What should Nighly do with this file?” Firefox dialogue window.

On your machine, PDF are associated to which application? Does it work normally with a fresh profile? https://support.mozilla.org/t5/Install-and-Update/Use-the-Profile-Manager-to-create-and-remove-Firefox-profiles/ta-p/2914

This is very likely a duplicate of bug 218257. If this is not the case, feel free to reopen.