Lost webRequest.OnBeforeRequest details.formData

In my extension app I use “webRequet.OnBeforeRequest”. “requestBody” of the “details” parameter is used, and when a large packet is transmitted, the value of “formData” is passed as an empty object. (Of course, the “raw” value of “requestBody” also exists, If the packet size is reduced, “formData” is delivered normally.) When transmitting the same data in Chrome, the data seems to be delivered normally without any problem. Is this a limitation of Firefox? Or is it a bug?
please check.