Are they PDFs? By default, Firefox will open PDFs in a tab. For servers that don't force a download, they open with their regular web address and are saved in the web content cache. For servers that force a download, they are saved in your downloads folder and then opened in a tab with a file:/// address. Do you want to change how Firefox handles PDFs? There are a lot of different options, from always using the web content cache to never accepting a forced download (you always get the choice). If it if some other kind of content that opens automatically, what is it? Does it open in Firefox or some other program? As a general reference on changing how Firefox handles downloads, see: [[Manage file types and download actions in Firefox]]