When nsITraceableChannel is used to insert JS-based nsIStreamListener's, off main thread delivery of OnDataAvailable is disabled. This is a potential performance consideration, and as such, users of nsITraceableChannel should be warned. To determine if deprecation is an option, this bug tracks adding telemetry to measure the usage of nsITraceableChannel in the wild. Note: Firebug is a popular addon that uses nsITraceableListener. Try to measure the percentage of cases in which Firebug is using the interface.

> Try to measure the percentage of cases in which Firebug is using the interface. As in, make sure we can tell what % of use cases are *not* using Firebug. For Firebug we already know that it only uses nsITraceableListener when actively debugging using the Net panel, and the perf hit is fine in that case. What we're worried about is if other addons are using it 100% of the time for regular browsing. (Steve already knows this--just putting it out there for folks reading at home :)

Bulk change to priority: https://bugzilla.mozilla.org/show_bug.cgi?id=1399258