UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Firefox/52.0

Steps to reproduce the problem:
1. Set up a WebRTC session, with video, negotiating the use of a=rtcp-fb:N ccm fir
2. Repeatedly send FIR messages with the same FIR sequence number, for more than twice the RTT

What is the expected behavior?
A keyframe is generated on the first receipt of the FIR; and then again after 2*RTT.  According to RFC 5104 Section 3.5.1.1:

If it
   receives repetitions of the FIR more than 2*RTT after it has sent a
   decoder refresh point, it shall send a new decoder refresh point.
   Two round trip times allow time for the decoder refresh point to
   arrive back to the requestor and for the end of repetitions of FIR to
   reach and be detected by the media sender.

What went wrong?
After it acts on the FIR message, Chrome WebRTC will ignore FIRs with the same sequence number forever after, until an FIR with a different sequence number is received.

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: <Copy from: 'about:version'>  Channel: n/a
OS Version: OS X 10.12
Flash Version: Shockwave Flash 25.0 r0

The relevant function is RTCPReceiver::HandleFir in webrtc/modules/rtp_rtcp/source/rtcp_receiver.cc .