STRs:

1. Open https://jsfiddle.net/jib1/ekjhr23t/

Expected result (like Chrome):

["0"]
["0"]

Actual result:

["0"]
[]

Regression range:

65:32.83 INFO: Narrowed nightly regression window from [2019-04-14, 2019-04-16] (2 days) to [2019-04-14, 2019-04-15] (1 days) (~0 steps left)
65:32.83 INFO: Got as far as we can go bisecting nightlies...
65:32.83 INFO: Last good revision: ec1f3a922d56e0a9393415872ee1d30c2f895fd7 (2019-04-14)
65:32.83 INFO: First bad revision: b8f49a14c458f56a6a08e4c177671108e386be3a (2019-04-15)
65:32.83 INFO: Pushlog:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=ec1f3a922d56e0a9393415872ee1d30c2f895fd7&tochange=b8f49a14c458f56a6a08e4c177671108e386be3a

This is a regression in the sense that something that used to fail now succeeds with incorrect behavior. This used to fail with:

["0"]
InvalidStateError: Cannot set remote offer or answer in current state have-remote-offer

IOW this has never worked correctly in Firefox, whereas in Chrome it works correctly.