This is a spinoff from legacy/trac#21396. While testing gk's fix for that bug, Kathy and I noticed messages like the following on the error console: 15:09:57.836 NS_ERROR_NOT_AVAILABLE: Component returned failure code: 0x80040111 (NS_ERROR_NOT_AVAILABLE) [nsIHttpChannel.getResponseHeader]1 content-policy.js:99:0 It turns out that this is caused by responses that have an HTTP status code of 304 ("Not Modified"); we saw them consistently when we reloaded the browserleaks.com page (which makes sense, since the browser sends conditional GET requests in that situation). These kinds of responses do not have a Location header. We should modify the Torbutton code in src/components/content-policy.js to add a try/catch around the call to `getResponseHeader()` and not log anything if the response's status code is 304 (we probably want to log something in all other cases). This is not urgent since it is not a new problem, but it would be good to fix it soon.