I would like to know if there's a way to force Firefox to ALWAYS open PDF links in browser, instead of allowing the website to force it's download. This is not something related to Adobe Reader plugin or PDF.js viewer or any other. Sometimes a link to a PDF makes it be opened in a plugin-container in the tab, ex.: http://www.empreende.org.br/pdf/Programas%20e%20Pol%C3%ADticas%20Sociais/Governan%C3%A7a,%20accountability%20e%20responsividade.pdf But sometimes it forces the PDF to be downloaded, like this one: http://ojs.uwindsor.ca/ojs/leddy/index.php/informal_logic/article/download/510/473 Can I prevent this to happen, i.e., can I tell Firefox to never download a PDF, always opening it in a tab, as usual, ignoring the "download" command?