SDP Unified Plan
Per spec, Firefox only supports unified plan and Chrome has now aggressively started moving towards it. Chrome has started supporting both, will move to unified plan by default soon and expects to completely drop support for Plan B by 2019.
The following email provides their expected timeline: https://groups.google.com/forum/#!searchin/discuss-webrtc/unified$20plan%7Csort:date/discuss-webrtc/9mVuJ7g3vgs/0yCYoRoxBAAJ
"End of April 2018: “Unified Plan” and the new APIs stabilizes. We encourage everyone to switch to UnifiedPlan semantics and the new APIs.
July 2018: Default SdpSemantics changes to UnifiedPlan. APIs listed under “Removed APIs” (except CreateSender) are removed at the same time.
No earlier than end of year 2018: PlanB semantics removed and UnifiedPlan becomes the only option."