Comment 2 for bug 752376

This appears to be because Software-Center doesn't accept file:/// URIs, which appears to be is the type of URI that Firefox passes to it, when you do "Open With: [Ubuntu Software Center]" in the file-download dialog.

Simpler steps to reproduce:
 1. Download a deb to e.g. /tmp/foo.deb
 2. Run in a terminal: "software-center file:///tmp/foo.deb"
 3. Close software-center
 4. Run in a terminal: software-center /tmp/foo.deb"

Step 2 is broken, step 4 works.