The internal components used to provide the webRequest filterResponseData feature (e.g. StreamFilter, StreamFilterParent and StreamFilterChild) are being executed in 3 different process (parent process, extension process and the child process where the web request is being intercepted and filtered) and coordinated with each other through IPC.

While investigating bugs, it would be useful if we could use MOZ_LOG environment variable to emit some debug logs that may help to trace what is going on across the 3 processes involved, especially when investigating issue that can't be reproduced if running under rr to record the execution.

As an example, while investigating Bug 1752655 I needed to introduce this kind of logging because the reduced test case I've attached to that issue changes behavior and doesn't trigger the same conditions anymore when running under rr.