As far as I can tell, it's only cancelled in #createNetworkEvent, but that isn't getting called at the right time. The correct time to cancel the blocked request would be in "http-on-modify-request" [here](https://searchfox.org/mozilla-central/rev/d81e60336d9f498ad3985491dc17c2b77969ade4/devtools/shared/network-observer/NetworkObserver.sys.mjs#472)
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As far as I can tell, it's only cancelled in #createNetworkEvent, but that isn't getting called at the right time. The correct time to cancel the blocked request would be in "http-on-modify-request" [here](https://searchfox.org/mozilla-central/rev/d81e60336d9f498ad3985491dc17c2b77969ade4/devtools/shared/network-observer/NetworkObserver.sys.mjs#472). Please let me know if that doesn't work.