--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -220,16 +220,24 @@
     "description": "Maximum number of concurrent threads reached during a given download session"
   },
   "BLOCKLIST_SYNC_FILE_LOAD": {
     "alert_emails": ["rvitillo@mozilla.com"],
     "expires_in_version": "35",
     "kind": "boolean",
     "description": "blocklist.xml has been loaded synchronously *** No longer needed (bug 1156565). Delete histogram and accumulation code! ***"
   },
+  "BOX_ALIGN_PROPS_IN_BLOCKS_FLAG": {
+    "alert_emails": ["bwerth@mozilla.com"],
+    "bug_numbers": [1351383],
+    "releaseChannelCollection": "opt-out",
+    "expires_in_version": "57",
+    "kind": "boolean",
+    "description": "Whether we have layed out any display:block containers with not-yet-supported properties from CSS Box Align."
+  },
   "CHECKERBOARD_DURATION": {
     "alert_emails": ["kgupta@mozilla.com"],
     "bug_numbers": [1238040],
     "expires_in_version": "58",
     "kind": "exponential",
     "high": 100000,
     "n_buckets": 50,
     "description": "Duration of a checkerboard event in milliseconds"