If you drag a tab, something calls getImageData on a canvas. The stack in the profiler looks like this: > [...] > mozilla::dom::CanvasRenderingContext2D::GetImageDataArray(JSContext*, int, int, unsigned int, unsigned int, JSObject**) > mozilla::dom::CanvasRenderingContext2D::GetImageData(JSContext*, double, double, double, double, mozilla::ErrorResult&) > mozilla::dom::CanvasRenderingContext2DBinding::getImageData(JSContext*, JS::Handle<JSObject*>, mozilla::dom::CanvasRenderingContext2D*, JSJitMethodCallArgs const&) > mozilla::dom::GenericBindingMethod(JSContext*, unsigned int, JS::Value*) > js::InternalCallOrConstruct(JSContext*, JS::CallArgs const&, js::MaybeConstruct) > <name omitted> > js::ForwardingProxyHandler::call(JSContext*, JS::Handle<JSObject*>, JS::CallArgs const&) const > js::CrossCompartmentWrapper::call(JSContext*, JS::Handle<JSObject*>, JS::CallArgs const&) const > js::Proxy::call(JSContext*, JS::Handle<JSObject*>, JS::CallArgs const&) > js::proxy_Call(JSContext*, unsigned int, JS::Value*) > js::InternalCallOrConstruct(JSContext*, JS::CallArgs const&, js::MaybeConstruct) > Interpret(JSContext*, js::RunState&) > js::RunScript(JSContext*, js::RunState&) > JS -> next > js::RunScript > js::InternalCallOrConstruct(JSContext*, JS::CallArgs const&, js::MaybeConstruct) > <name omitted> > js::CallSelfHostedFunction(JSContext*, JS::Handle<js::PropertyName*>, JS::Handle<JS::Value>, js::AnyInvokeArgs const&, JS::MutableHandle<JS::Value>) > AsyncFunctionResume(JSContext*, JS::Handle<js::PromiseObject*>, JS::Handle<JS::Value>, ResumeKind, JS::Handle<JS::Value>) > PromiseReactionJob(JSContext*, unsigned int, JS::Value*) > js::InternalCallOrConstruct(JSContext*, JS::CallArgs const&, js::MaybeConstruct) > <name omitted> > JS::Call(JSContext*, JS::Handle<JS::Value>, JS::Handle<JS::Value>, JS::HandleValueArray const&, JS::MutableHandle<JS::Value>) > mozilla::dom::PromiseJobCallback::Call(JSContext*, JS::Handle<JS::Value>, mozilla::ErrorResult&) > mozilla::PromiseJobRunnable::Run() > mozilla::dom::Promise::PerformMicroTaskCheckpoint() > mozilla::CycleCollectedJSContext::AfterProcessTask(unsigned int) > XPCJSContext::AfterProcessTask(unsigned int) > nsThread::ProcessNextEvent(bool, bool*) > [...] This stack contains lots of C++ functions, a few pseudo frames, and one (1) JS frame. The JS frame is for the self-hosted "next" function. There are at least two JS functions missing in this call stack: The one that calls next, and the one that calls getImageData. It looks like it would be the JS interpreter's job to push pseudo frames for these functions, and it doesn't do it.