Within Bug 1188538 we slightly modify test coverage for

https://searchfox.org/mozilla-central/rev/6cd54550a27e2f6ca0755a25328f769e41e524f4/browser/components/extensions/test/browser/browser_ext_search_favicon.js

Quoting MixedPuppys comment from phabricator review of Bug 1188538:

Looking at this test file, and comparing against the code paths in ExtensionProtocolHandler, it seems we're missing a couple things. The test set should be:

favicon_url: "goodIcon.png" <-- not explicitly tested here, but search relies on it working
favicon_url: "iDoNotExist.png" <-- what the test now does
favicon_url: "moz-extension://invalidOtherExtensionUUID/iDoNotExist.png" <-- your patch fixes, but test removed
favicon_url: "moz-extension://validOtherExtensionUUID/notWhitelisted.png" <-- not tested here (perhaps elsewhere)
favicon_url: "moz-extension://validOtherExtensionUUID/whitelisted.png" <-- not tested here (perhaps elsewhere)