1. Load the attached extension. 2. The extension opens a tab with instructions, follow those instructions and you will basically try different ways of running chrome.downloads.download({url:'blob:moz-extension://...'}); where the URL is actually created using URL.createObjectURL(blob), and one <a download> test. Expected: At each of the four attempt, a download dialog should appear. Actual (Firefox 47.0.1 and Firefox Nightly (50)): - The chrome.downloads.download invocation fails with: Error: Type error for parameter options (Error processing url: Error: Access denied for URL blob:moz-extension://9089d5f0-2813-4a73-b5aa-733b8d777294/6e0c929e-3e68-46ab-870c-894bec0df00b) for downloads.download. - The dialog appears as expected when I use <a download> (unless e10s is enabled - see bug 1287346). The failure originates from http://searchfox.org/mozilla-central/rev/bfcc10319e4e3ce78367fa9bba9316f7eb5248b6/toolkit/components/extensions/Schemas.jsm#140 |this.principal| references |this.params.principal|, but the latter is not set. http://searchfox.org/mozilla-central/rev/bfcc10319e4e3ce78367fa9bba9316f7eb5248b6/toolkit/components/extensions/Schemas.jsm#137 ... because Schema.inject has callers that don't set the principal on |wrapperFunc|. http://searchfox.org/mozilla-central/rev/bfcc10319e4e3ce78367fa9bba9316f7eb5248b6/toolkit/components/extensions/Schemas.jsm#1618

This should work since bug 1271345, and we definitely have tests for it.

Just tested again on the latest build, it works. I guess that I only tested with 47 and an old Nightly 49, and not 50 itself. Sorry for the noise.