--- a/toolkit/components/aboutmemory/content/aboutMemory.js
+++ b/toolkit/components/aboutmemory/content/aboutMemory.js
@@ -58,16 +58,20 @@ const UNITS_PERCENTAGE       = Ci.nsIMem
 
 // Because about:memory and about:compartments are non-standard URLs,
 // location.search is undefined, so we have to use location.href here.
 const gVerbose = location.href === "about:memory?verbose" ||
                  location.href === "about:compartments?verbose";
 
 let gChildMemoryListener = undefined;
 
+// This is a useful function and an efficient way to implement it.
+String.prototype.startsWith =
+  function(s) { return this.lastIndexOf(s, 0) === 0; }
+
 //---------------------------------------------------------------------------
 
 // Forward slashes in URLs in paths are represented with backslashes to avoid
 // being mistaken for path separators.  Paths/names where this hasn't been
 // undone are prefixed with "unsafe"; the rest are prefixed with "safe".
 function flipBackslashes(aUnsafeStr)
 {
   return aUnsafeStr.replace(/\\/g, '/');
@@ -96,20 +100,20 @@ function addChildObserversAndUpdate(aUpd
   gChildMemoryListener = aUpdateFn;
   os.addObserver(gChildMemoryListener, "child-memory-reporter-update", false);
  
   gChildMemoryListener();
 }
 
 function onLoad()
 {
-  if (location.href.indexOf("about:memory") === 0) {
+  if (location.href.startsWith("about:memory")) {
     document.title = "about:memory";
     onLoadAboutMemory();
-  } else if (location.href.indexOf("about:compartment") === 0) {
+  } else if (location.href.startsWith("about:compartment")) {
     document.title = "about:compartments";
     onLoadAboutCompartments();
   } else {
     assert(false, "Unknown location");
   }
 }
 
 function onUnload()
@@ -447,32 +451,32 @@ Report.prototype = {
       this._amount = r._amount;
     }
     this._nMerged = this._nMerged ? this._nMerged + 1 : 2;
   },
 
   treeNameMatches: function(aTreeName) {
     // Nb: the '/' must be present, because we have a KIND_OTHER reporter
     // called "explicit" which is not part of the "explicit" tree.
-    return this._unsafePath.indexOf(aTreeName) === 0 &&
+    return this._unsafePath.startsWith(aTreeName) &&
            this._unsafePath.charAt(aTreeName.length) === '/';
   }
 };
 
 function getReportsByProcess(aMgr)
 {
   // Ignore the "smaps" multi-reporter in non-verbose mode, and the
   // "compartments" multi-reporter all the time.  (Note that reports from these
   // multi-reporters can reach here as single reports if they were in the child
   // process.)
 
   function ignoreSingle(aPath) 
   {
-    return (aPath.indexOf("smaps/") === 0 && !gVerbose) ||
-           (aPath.indexOf("compartments/") === 0)
+    return (aPath.startsWith("smaps/") && !gVerbose) ||
+           (aPath.startsWith("compartments/"))
   }
 
   function ignoreMulti(aName)
   {
     return ((aName === "smaps" && !gVerbose) ||
             (aName === "compartments"));
   }
 
@@ -1520,17 +1524,17 @@ Compartment.prototype = {
 function getCompartmentsByProcess(aMgr)
 {
   // Ignore anything that didn't come from the "compartments" multi-reporter.
   // (Note that some such reports can reach here as single reports if they were
   // in the child process.)
 
   function ignoreSingle(aPath) 
   {
-    return aPath.indexOf("compartments/") !== 0;
+    return !aPath.startsWith("compartments/");
   }
 
   function ignoreMulti(aName)
   {
     return aName !== "compartments";
   }
 
   let compartmentsByProcess = {};
@@ -1555,17 +1559,17 @@ function getCompartmentsByProcess(aMgr)
 
     } else if (unsafeNames[0] === "compartments" && unsafeNames[1] == "user" &&
         unsafeNames.length == 3)
     {
       isSystemCompartment = false;
       // These null principal compartments are user compartments according to
       // the JS engine, but they look odd being shown with content
       // compartments, so we put them in the system compartments list.
-      if (unsafeNames[2].indexOf("moz-nullprincipal:{") === 0) {
+      if (unsafeNames[2].startsWith("moz-nullprincipal:{")) {
         isSystemCompartment = true;
       }
 
     } else {
       assert(false, "bad compartments path: " + aUnsafePath);
     }
     let c = new Compartment(unsafeNames[2], isSystemCompartment);