TL;DR: Currently, Firefox does not enforce the transactionId mechanic in 
RTCRtpSender.setParameters as specified in webrtc-pc. This means that right 
now, content code is able to call setParameters without calling 
getParameters first. However, once transactionId is implemented, this type 
of code will no longer work. 

Spec summary: 
https://w3c.github.io/webrtc-pc/#dom-rtcrtpsender-setparameters 
https://w3c.github.io/webrtc-pc/#dom-rtcrtpsender-getparameters 

webrtc-pc specifies that RTCRtpSender.setParameters and 
RTCRtpSender.getParameters use RTCRtpSendParameters.transactionId to 
implement a transaction mechanism. The idea is that content calls 
getParameters to get the current parameters, which will contain a new 
unique transactionId. Content then modifies those parameters, and calls 
setParameters with the modified parameters, which will include the unique 
transactionId. If the transactionId does not match the value from the most 
recent call to getParameters, the setParameters call fails with an 
InvalidModificationError. 

Current state: 
Currently, our implementation of RTCRtpSender.setParameters does not 
enforce this transaction mechanic, which allows setParameters to be called 
with an entirely content-created RTCRtpSendParameters. Any content code 
that does this will stop working. 

Best regards, 
Byron Campen 

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