Created attachment 8885226 [details] test.xpi Steps to reproduce: 1. Go to about:debugging and install this 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 open a blob: URL" link on the options page (linking to a blob:moz-extension:// URL). Expected results: The blob URL opens and JSON viewer kicks in. This is what happens if you open the same page via a file:/// URL or if you middle-click the link to open it in a new tab. Actual results: The page turns blank without changing URL in the location bar, the load indicator keeps spinning forever. I've tested in Firefox 56.0a1 nightly (2017-07-11) on Ubuntu. This is very similar to bug 1303868, but with the steps listed there the issue is not reproducible for me.
This still fails with extensions.webextensions.remote=false, but works with extensions.webextensions.remote=true, not certain it's worth addressing since default will become remote.
I used the test addon from bug 1331176 to test in the sidebar and a tab (browserAction button opens the page in a tab).
Verified test with addon attached here as well, same results.