I ran mozregression like this (twice, to be sure):

MOZ_ENABLE_WAYLAND=1 mozregression --profile /home/jani/.mozilla/firefox/fyaus28k.LP1940417 --bad 91 --good 89 --profile-persistence clone-first

Here’s the result (it was the same for both bisects):

5:25.73 INFO: No more integration revisions, bisection finished.
5:25.73 INFO: Last good revision: 9f0fbb1431721c9eae68a3c94ae49a4d33fdb1f8
5:25.73 INFO: First bad revision: 7b82d177a6b979036f180329be6b029d690d9e0c
5:25.73 INFO: Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=9f0fbb1431721c9eae68a3c94ae49a4d33fdb1f8&tochange=7b82d177a6b979036f180329be6b029d690d9e0c

According to that log, this changeset was introduced to address #1661516.