MCVE for Firefox behavior on redirects to local file URLs

Node.js and NPM are required.

1. Install dependencies using npm install.
2. Change the absolute paths in the href of the 'Direct link' in index.html and in the redirect handler in index.js according to your local repository path of the file test.pdf.
3. Start the minimal web server using node index.js.
4. Add http://localhost:8080/ to the Firefox LocalFileLinks (in policies.json).
5. Use about:config in Firefox to set network.protocol-handler.expose.file to false and network.protocol-handler.external.file to true.
6. Use the Firefox settings to set the default action for PDF files to "Open in Firefox".
7. Firefox might need a restart (at least for the changes in step 4).
8. In Firefox, navigate to http://localhost:8080.
9. Click on the link that says "Direct link". Firefox will open the file test.pdf using the default PDF viewer of the system (e.g. Adobe Reader).
10. Click on the link that says "Link with redirect". Firefox will open the file test.pdf using the builtin PDF viewer instead ot the default PDF viewer of the system.