While it may seem silly to put a send simulcast attr in a recvonly or inactive m-section (or recv in a sendonly/inactive), it is not invalid, and webrtc-pc is written such that removing the simulcast attr will result in permanently disabling simulcast, which is not what we want.
This code needs to be removed:
https://github.com/mozilla/webrtc-sdp/blob/master/src/lib.rs#L701-L721
While it may seem silly to put a send simulcast attr in a recvonly or inactive m-section (or recv in a sendonly/inactive), it is not invalid, and webrtc-pc is written such that removing the simulcast attr will result in permanently disabling simulcast, which is not what we want.
Here's the bug on the Firefox side: https://bugzilla.mozilla.org/show_bug.cgi?id=1807193