Is this problem on Android or on a desktop version of Firefox? I'm guessing it's Windows, MacOS, or Linux? Firefox on desktop can do a few different things when retrieving a PDF: (1) Open it directly into the PDF viewer without asking questions. Firefox saves the PDF in the web cache, which isn't directly accessible to you. Use the Download button in the PDF viewer in this case to save the file to a more convenient location. (2) Show a Download dialog with 3 choices Here you can choose to open the PDF in Firefox or in an outside application. In either of those cases, Firefox saves the PDF in the system Temp folder. This folder is subject to getting cleared. Alternately, you can instruct Firefox to Save the file. This file is saved in your designated Downloads folder or if you set Firefox to let you choose where to save, wherever you chose.