--- a/toolkit/components/aboutmemory/content/aboutMemory.js
+++ b/toolkit/components/aboutmemory/content/aboutMemory.js
@@ -397,19 +397,19 @@ function onLoad() {
   appendElementWithText(gFooter, "div", "legend hiddenOnMobile", legendText2);
 
   // See if we're loading from a file.  (Because about:memory is a non-standard
   // URL, location.search is undefined, so we have to use location.href
   // instead.)
   let search = location.href.split("?")[1];
   if (search) {
     let searchSplit = search.split("&");
-    for (let i = 0; i < searchSplit.length; i++) {
-      if (searchSplit[i].toLowerCase().startsWith("file=")) {
-        let filename = searchSplit[i].substring("file=".length);
+    for (let s of searchSplit) {
+      if (s.toLowerCase().startsWith("file=")) {
+        let filename = s.substring("file=".length);
         updateAboutMemoryFromFile(decodeURIComponent(filename));
         return;
       }
     }
   }
 }
 
 // ---------------------------------------------------------------------------
@@ -561,18 +561,17 @@ function updateAboutMemoryFromJSONObject
                 "data version number missing or doesn't match");
     assertInput(aObj.hasMozMallocUsableSize !== undefined,
                 "missing 'hasMozMallocUsableSize' property");
     assertInput(aObj.reports && aObj.reports instanceof Array,
                 "missing or non-array 'reports' property");
 
     let processMemoryReportsFromFile =
         function(aHandleReport, aDisplayReports) {
-      for (let i = 0; i < aObj.reports.length; i++) {
-        let r = aObj.reports[i];
+      for (let r of aObj.reports) {
         aHandleReport(r.process, r.path, r.kind, r.units, r.amount,
                       r.description, r._presence);
       }
       aDisplayReports();
     };
     appendAboutMemoryMain(processMemoryReportsFromFile,
                           aObj.hasMozMallocUsableSize);
   } catch (ex) {
@@ -767,19 +766,17 @@ DReport.ADDED_FOR_BALANCE = 3;
  * DReport objects for values.
  *
  * @param aJSONReports
  *        The |reports| field of a JSON object.
  * @return The constructed report map.
  */
 function makeDReportMap(aJSONReports) {
   let dreportMap = {};
-  for (let i = 0; i < aJSONReports.length; i++) {
-    let jr = aJSONReports[i];
-
+  for (let jr of aJSONReports) {
     assert(jr.process !== undefined, "Missing process");
     assert(jr.path !== undefined, "Missing path");
     assert(jr.kind !== undefined, "Missing kind");
     assert(jr.units !== undefined, "Missing units");
     assert(jr.amount !== undefined, "Missing amount");
     assert(jr.description !== undefined, "Missing description");
 
     // Strip out some non-deterministic stuff that prevents clean diffs.
@@ -1040,18 +1037,17 @@ function appendAboutMemoryMain(aProcessR
       if (aProcessA > aProcessB) {
         return 1;
       }
 
       return 0;
     });
 
     // Generate output for each process.
-    for (let i = 0; i < processes.length; i++) {
-      let process = processes[i];
+    for (let [i, process] of processes.entries()) {
       let section = appendElement(gMain, "div", "section");
 
       appendProcessAboutMemoryElements(section, i, process,
                                        pcollsByProcess[process]._trees,
                                        pcollsByProcess[process]._degenerates,
                                        pcollsByProcess[process]._heapTotal,
                                        aHasMozMallocUsableSize);
     }
@@ -1087,19 +1083,19 @@ function TreeNode(aUnsafeName, aUnits, a
   // - _description
   // - _hideKids (only defined if true)
   // - _maxAbsDescendant (on-demand, only when gIsDiff is set)
 }
 
 TreeNode.prototype = {
   findKid(aUnsafeName) {
     if (this._kids) {
-      for (let i = 0; i < this._kids.length; i++) {
-        if (this._kids[i]._unsafeName === aUnsafeName) {
-          return this._kids[i];
+      for (let kid of this._kids) {
+        if (kid._unsafeName === aUnsafeName) {
+          return kid;
         }
       }
     }
     return undefined;
   },
 
   // When gIsDiff is false, tree operations -- sorting and determining if a
   // sub-tree is significant -- are straightforward. But when gIsDiff is true,
@@ -1115,18 +1111,18 @@ TreeNode.prototype = {
 
     if ("_maxAbsDescendant" in this) {
       // We've computed this before? Return the saved value.
       return this._maxAbsDescendant;
     }
 
     // Compute the maximum absolute value of all descendants.
     let max = Math.abs(this._amount);
-    for (let i = 0; i < this._kids.length; i++) {
-      max = Math.max(max, this._kids[i].maxAbsDescendant());
+    for (let kid of this._kids) {
+      max = Math.max(max, kid.maxAbsDescendant());
     }
     this._maxAbsDescendant = max;
     return max;
   },
 
   toString() {
     switch (this._units) {
       case UNITS_BYTES: return formatBytes(this._amount);
@@ -1195,18 +1191,18 @@ function fillInTree(aRoot) {
         aT._nMerged = kid._nMerged;
       }
       assert(!aT._hideKids && !kid._hideKids, "_hideKids set when merging");
 
     } else {
       // Non-leaf node with multiple children.  Derive its _amount and
       // _description entirely from its children...
       let kidsBytes = 0;
-      for (let i = 0; i < aT._kids.length; i++) {
-        kidsBytes += fillInNonLeafNodes(aT._kids[i]);
+      for (let kid of aT._kids) {
+        kidsBytes += fillInNonLeafNodes(kid);
       }
 
       // ... except in one special case. When diffing two memory report sets,
       // if one set has a node with children and the other has the same node
       // but without children -- e.g. the first has "a/b/c" and "a/b/d", but
       // the second only has "a/b" -- we need to add a fake node "a/b/(fake)"
       // to the second to make the trees comparable. It's ugly, but it works.
       if (aT._amount !== undefined &&
@@ -1295,18 +1291,18 @@ function sortTreeAndInsertAggregateNodes
 
   aT._kids.sort(TreeNode.compareAmounts);
 
   // If the first child is insignificant, they all are, and there's no point
   // creating an aggregate node that lacks siblings.  Just set the parent's
   // _hideKids property and process all children.
   if (isInsignificant(aT._kids[0])) {
     aT._hideKids = true;
-    for (let i = 0; i < aT._kids.length; i++) {
-      sortTreeAndInsertAggregateNodes(aTotalBytes, aT._kids[i]);
+    for (let kid of aT._kids) {
+      sortTreeAndInsertAggregateNodes(aTotalBytes, kid);
     }
     return;
   }
 
   // Look at all children except the last one.
   let i;
   for (i = 0; i < aT._kids.length - 1; i++) {
     if (isInsignificant(aT._kids[i])) {
@@ -1327,18 +1323,18 @@ function sortTreeAndInsertAggregateNodes
       aggT._amount = aggBytes;
       aggT._description =
         nAgg + " sub-trees that are below the " + kSignificanceThresholdPerc +
         "% significance threshold.";
       aT._kids.splice(i0, nAgg, aggT);
       aT._kids.sort(TreeNode.compareAmounts);
 
       // Process the moved children.
-      for (i = 0; i < aggT._kids.length; i++) {
-        sortTreeAndInsertAggregateNodes(aTotalBytes, aggT._kids[i]);
+      for (let kid of aggT._kids) {
+        sortTreeAndInsertAggregateNodes(aTotalBytes, kid);
       }
       return;
     }
 
     sortTreeAndInsertAggregateNodes(aTotalBytes, aT._kids[i]);
   }
 
   // The first n-1 children were significant.  Don't consider if the last child
@@ -1482,23 +1478,21 @@ function appendProcessAboutMemoryElement
       maxStringLength = length;
     }
     otherDegenerates.push(t);
   }
   otherDegenerates.sort(TreeNode.compareUnsafeNames);
 
   // Now generate the elements, putting non-degenerate trees first.
   let pre = appendSectionHeader(aP, "Other Measurements");
-  for (let i = 0; i < otherTrees.length; i++) {
-    let t = otherTrees[i];
+  for (let t of otherTrees) {
     appendTreeElements(pre, t, aProcess, "");
     appendTextNode(pre, "\n"); // blank lines after non-degenerate trees
   }
-  for (let i = 0; i < otherDegenerates.length; i++) {
-    let t = otherDegenerates[i];
+  for (let t of otherDegenerates) {
     let padText = pad("", maxStringLength - t.toString().length, " ");
     appendTreeElements(pre, t, aProcess, padText);
   }
   appendTextNode(aP, "\n"); // gives nice spacing when we copy and paste
 
   // Add any warnings about inaccuracies in the "explicit" tree due to platform
   // limitations.  These must be computed after generating all the text.  The
   // newlines give nice spacing if we copy+paste into a text buffer.
@@ -1887,18 +1881,17 @@ function appendTreeElements(aP, aRoot, a
       let tlThisForMost, tlKidsForMost;
       if (aT._kids.length > 1) {
         tlThisForMost = aTlKids + "├──";
         tlKidsForMost = aTlKids + "│  ";
       }
       let tlThisForLast = aTlKids + "└──";
       let tlKidsForLast = aTlKids + "   ";
 
-      for (let i = 0; i < aT._kids.length; i++) {
-        let kid = aT._kids[i];
+      for (let [i, kid] of aT._kids.entries()) {
         let isLast = i == aT._kids.length - 1;
         aUnsafeNames.push(kid._unsafeName);
         appendTreeElements2(d, aProcess, aUnsafeNames, aRoot, kid,
                             !isLast ? tlThisForMost : tlThisForLast,
                             !isLast ? tlKidsForMost : tlKidsForLast,
                             valueText.length);
         aUnsafeNames.pop();
       }