diff --git a/devtools/client/locales/en-US/netmonitor.properties b/devtools/client/locales/en-US/netmonitor.properties
index 75f0e0b62e57..e6f6b57dc43a 100644
--- a/devtools/client/locales/en-US/netmonitor.properties
+++ b/devtools/client/locales/en-US/netmonitor.properties
@@ -219,6 +219,12 @@ networkMenu.sizeServiceWorker=service worker
 # in the network menu specifying the time for a request to finish (in milliseconds).
 networkMenu.totalMS=→ %S ms
 
+# This string is used to concatenate tooltips (netmonitor.waterfall.tooltip.*)
+# in the requests waterfall for total time (in milliseconds). \\u0020 represents
+# a whitespace. You can replace this with a different character, e.g. an hyphen
+# or a period, if a comma doesn't work for your language.
+netmonitor.waterfall.tooltip.separator=,\u0020
+
 # LOCALIZATION NOTE (netmonitor.waterfall.tooltip.total): This is part of the tooltip
 # displayed in the requests waterfall for total time (in milliseconds).
 netmonitor.waterfall.tooltip.total=Total %S ms
diff --git a/devtools/client/netmonitor/src/components/request-list-column-waterfall.js b/devtools/client/netmonitor/src/components/request-list-column-waterfall.js
index a6277655c146..1fdf07b7eeb0 100644
--- a/devtools/client/netmonitor/src/components/request-list-column-waterfall.js
+++ b/devtools/client/netmonitor/src/components/request-list-column-waterfall.js
@@ -96,7 +96,10 @@ function timingBoxes(item) {
     tooltip.push(L10N.getFormatStr("netmonitor.waterfall.tooltip.total", totalTime));
   }
 
-  return { boxes, tooltip: tooltip.join(", ") };
+  return {
+    boxes,
+    tooltip: tooltip.join(L10N.getStr("netmonitor.waterfall.tooltip.separator"))
+  };
 }
 
 module.exports = RequestListColumnWaterfall;