Prevent PDF Viewer on Download von Morpheus0x

This extension prevents a new tab from opening after downloading a PDF and optionally shows a `Save As...` window to choose where to save the PDF.

Due to changes in Firefox how PDF files are handled, any downloaded PDF will instantly open in a new tab by default. This extension restore how PDFs were handled before Firefox 98.0.

This extension works nearly 100%, the only issue is with PDF "downloads" directly from a web application that uses blob to store the PDF before downloading it. In this case only the opening of a new tab with the PDF Viewer is prevented, since there isn't an actual http request that can be intercepted.