Hi kkayser, Firefox handles bookmarked URLs and links the same way, so you can direct Firefox to open all bookmarked URLs and links to PDFs to open in your preferred external viewer. This article has the steps:

View PDF files using Firefox’s built-in viewer

That works with files directly clicked, but not with bookmarks. The only way I can get a bookmarked file to open with Acrobat is to find it with Windows explorer and open it there.

There have been issues reported with opening PDF files from the download manager window, so maybe bookmarks are affected as well.

• Bug 1653807 - PDF handling from Download panel is broken (no longer uses OS handler)

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)

You create bookmarks to locally saved files? Apparently the rules for how to handle web content don't apply to bookmarks to locally saved PDFs like file:///C:/Users/username/Downloads/test.pdf

If you never ever want to use the PDF Viewer, you have the option to fully disable it:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste pdfjs and pause while the list is filtered

(3) Double-click the pdfjs.disabled preference to switch the value from false to true

More info on about:config: Configuration Editor for Firefox.

You could file a new bug:

• https://bugzilla.mozilla.org/
• See: Bugzilla Etiquette