--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -3891,17 +3891,17 @@ nsLayoutUtils::PaintFrame(gfxContext* aR
         pt->dlMs() = retainedBuildTime;
         pt->dl2Ms() = fullBuildTime;
       } else if (pt) {
         pt->dlMs() = (TimeStamp::Now() - dlTotalStart).ToMilliseconds();
       }
     }
 
     if (stats.triedPartial && gfxPrefs::LayoutDisplayListBuildTwice()) {
-      printf(R"({ "retained": %.3f, "full": %.3f, "modifiedFrames": %lu, )"
+      printf(R"({ "retained": %.3f, "full": %.3f, "modifiedFrames": %zu, )"
              R"("merged": %d, "items": { )"
              R"("list": %u, "old": %u, "new": %u, "reused": %u } },)" "\n",
         retainedBuildTime, fullBuildTime, stats.frames.Length(), stats.merged,
         stats.totalItems, stats.oldItems, stats.newItems, stats.reusedItems);
 
 #if 0
       printf("Modified frames:\n");
       for (nsIFrame* frame : stats.frames) {