The menu-vertical binding (bound to menulist[type=description]) appears to be unused in m-c: https://searchfox.org/mozilla-central/search?q=type%3D%22description%22&path=xul The binding, along with any associated CSS can be removed: https://searchfox.org/mozilla-central/search?q=menulist-description&path=. Note that there is a single consumer in comm-central, so either that will have to become a plain menulist or the binding will need to be migrated there to continue working: https://searchfox.org/comm-central/search?q=type%3D%22description%22&path=xul. It appears that the difference (some extra flex and different styling) could be migrated as CSS-only and not require porting the binding.

https://treeherder.mozilla.org/#/jobs?repo=try&revision=8b1c824f27198f053c25af782664db316cdf0079

Orange looks unrelated: https://treeherder.mozilla.org/#/jobs?repo=try&revision=bfc207ab55685a23d863b6779ee7a3c6f8f7ceef

Pushed by mozilla@noorenberghe.ca: https://hg.mozilla.org/integration/mozilla-inbound/rev/efb319fac7c3 Removed the unused menulist-description binding r=bgrins

Moving to Core:XUL per https://bugzilla.mozilla.org/show_bug.cgi?id=1455336