User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:65.0) Gecko/20100101 Firefox/65.0 Steps to reproduce: On this site https://www.seko.se/branscher/trafik/sekogreencargo/avtal/ i went to this link https://www.seko.se/siteassets/forhandlings-branschorganisationer/seko-greencargo/pdf-er/avtal/allmanna-anstvillkor-o-reseavtal-2018-seko-utgava-20.pdf Actual results: Had to force shut down nighly and restart it Expected results: open a .PDF dokument. (It worked in edge!) @nightly-community

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/