When you open a saved file from the Downloads panel, Firefox usually looks to your OS-level association for the content. If the file has a .pdf extension on the name, that normally would be some kind of PDF reader and not VSCode. But some sites may not send a full file name, or Firefox might save under a script name such as pushpdf.php in which case it would make more sense that it opens in VSCode. When you open content from a link in a page, Firefox takes its cue from the Content-Type header sent with the content instead of from the file extension. So if the site sends '''application/pdf''' Firefox will, by default, use the built-in PDF viewer regardless of whether it's a .pdf, .jsp, etc.