Writing a test for bug 1323036, read this: > If you want to see the headers that are actually sent, without the risk that > another add-on will subsequently alter them, use onSendHeaders, although you > can't modify headers on this event. from https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/webRequest/onBeforeSendHeaders This doesn't seem to be true.
I'm pretty sure we have tests for this.
You are right, of course. I likely added a second onBeforeSendHeaders listener instead, exactly as I've done just now, while trying to reproduce this. (It's a perfect mistake -- it behaved exactly as expected from onSendHeaders, except for the observed "bug" :/)