Comment 5 for bug 1042771

This bug also exists in Trusty and with the proliferation of the sanitized helper this is rather concerning.

Note: it's possible Firefox will open the PDF with its builtin reader when following the steps to reproduce. If that's the case, you simply need to click the "Download" button and do "Open with" and pick the "Document Viewer (default)" choice. I know that by default the builtin PDF is protected by Apparmor but this is just an example of how the sanitized helper can lead to lower security.