Although the stream-filtering API works, and the web content sees the correct result, inspecting the network requests in the devtools (in the network or console views) shows the original response rather than the altered one. This makes it difficult to debug the responses (and rather easy to get confused).
That's because developer tools (I believe) does its network monitoring in the parent process, before any stream filters are applied, while we do ours in the child, after.
Component: WebExtensions: Request Handling → Developer Tools: Netmonitor
Product: Toolkit → Firefox
Thanks for the report! Honza
Priority: -- → P3
status-firefox57: --- → fix-optional
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