It looks like Firefox is no longer specifying a valid IP address in the `c=` line in its SDP. The c line specifies where the remote party should send media. Leaving this as 0.0.0.0 *should* be okay, as ICE negotiation will take precedence. Any IP address specified in the c line would serve as a fallback, I believe, in case the other endpoint doesn't support ICE. FreeSWITCH (and I'm guessing other endpoints as well) sees this 0.0.0.0 IP address and assumes that Firefox doesn't want to receive any media. Since FreeSWITCH does support ICE negotiation, I would count this as a FreeSWITCH bug. It seems odd that Firefox would change this now, though.
In any case, looks like this calls for an addition to our Hacks file. This is the second "a=sendrecv vs c=..." conflict we've had, so our behavior, browsers' behavior, and servers' behavior may be up for a full review. In any case, we'll be sure to get this sorted ahead of Firefox Beta hitting production, likely in the form of a hackCLineIP parameter or something.
Stay tuned for updates...