Created attachment 8885231 [details] Test extension Steps to reproduce: 1. Go to about:debugging and install the attached test extension as a temporary add-on. 2. Go to about:addons, find "WebExtensions test" in the list and click Preferences. 3. Click "Click here to download some data" link on the options page (linking to a data: URL, with download attribute set). Expected results: Firefox offers to download the data, which is what I see when opening the same page from a file:/// URL. Actual results: Firefox opens the URL in the browser. I've tested in Firefox 56.0a1 nightly (2017-07-11) on Ubuntu. Note that, according to https://developer.mozilla.org/en/docs/Web/HTML/Element/a#attr-download, the download attribute is only respected for same-origin URLs. However, blob: and data: URLs are explicitly considered same-origin here so this shouldn't prevent showing the download prompt. I assume that the same issue exists for blob: URL but cannot test due to bug 1379954.
Placing the link inside an about:blank frame within the extension page allows working around this issue, downloading works for both data: and blob: URLs then.
There's a dupe somewhere I'm sure, but I can't find it.
(In reply to Andy McKay [:andym] from comment #2) > There's a dupe somewhere I'm sure, but I can't find it. I'm pretty sure it's bug 1331176, I just haven't looked in detail yet.
Bug 1331176 was fixed in nightly as of 2017-06-30, the bug I filed here was reproduced in 2017-07-11. Related - quite likely. Duplicate - no, definitely not.