diff --git a/toolkit/devtools/sourcemap/SourceMap.jsm b/toolkit/devtools/sourcemap/SourceMap.jsm
index eefdd4dc24b9..c28d2585cf2d 100644
--- a/toolkit/devtools/sourcemap/SourceMap.jsm
+++ b/toolkit/devtools/sourcemap/SourceMap.jsm
@@ -85,36 +85,11 @@ define('source-map/source-map-consumer', ['require', 'exports', 'module' ,  'sou
     // #72 and bugzil.la/889492.
     this._names = ArraySet.fromArray(names, true);
     this._sources = ArraySet.fromArray(sources, true);
+
     this.sourceRoot = sourceRoot;
     this.sourcesContent = sourcesContent;
+    this._mappings = mappings;
     this.file = file;
-
-    // `this._generatedMappings` and `this._originalMappings` hold the parsed
-    // mapping coordinates from the source map's "mappings" attribute. Each
-    // object in the array is of the form
-    //
-    //     {
-    //       generatedLine: The line number in the generated code,
-    //       generatedColumn: The column number in the generated code,
-    //       source: The path to the original source file that generated this
-    //               chunk of code,
-    //       originalLine: The line number in the original source that
-    //                     corresponds to this chunk of generated code,
-    //       originalColumn: The column number in the original source that
-    //                       corresponds to this chunk of generated code,
-    //       name: The name of the original symbol which generated this chunk of
-    //             code.
-    //     }
-    //
-    // All properties except for `generatedLine` and `generatedColumn` can be
-    // `null`.
-    //
-    // `this._generatedMappings` is ordered by the generated positions.
-    //
-    // `this._originalMappings` is ordered by the original positions.
-    this._generatedMappings = [];
-    this._originalMappings = [];
-    this._parseMappings(mappings, sourceRoot);
   }
 
   /**
@@ -135,9 +110,9 @@ define('source-map/source-map-consumer', ['require', 'exports', 'module' ,  'sou
                                                               smc.sourceRoot);
       smc.file = aSourceMap._file;
 
-      smc._generatedMappings = aSourceMap._mappings.slice()
+      smc.__generatedMappings = aSourceMap._mappings.slice()
         .sort(util.compareByGeneratedPositions);
-      smc._originalMappings = aSourceMap._mappings.slice()
+      smc.__originalMappings = aSourceMap._mappings.slice()
         .sort(util.compareByOriginalPositions);
 
       return smc;
@@ -159,9 +134,66 @@ define('source-map/source-map-consumer', ['require', 'exports', 'module' ,  'sou
     }
   });
 
+  // `__generatedMappings` and `__originalMappings` are arrays that hold the
+  // parsed mapping coordinates from the source map's "mappings" attribute. They
+  // are lazily instantiated, accessed via the `_generatedMappings` and
+  // `_originalMappings` getters respectively, and we only parse the mappings
+  // and create these arrays once queried for a source location. We jump through
+  // these hoops because there can be many thousands of mappings, and parsing
+  // them is expensive, so we only want to do it if we must.
+  //
+  // Each object in the arrays is of the form:
+  //
+  //     {
+  //       generatedLine: The line number in the generated code,
+  //       generatedColumn: The column number in the generated code,
+  //       source: The path to the original source file that generated this
+  //               chunk of code,
+  //       originalLine: The line number in the original source that
+  //                     corresponds to this chunk of generated code,
+  //       originalColumn: The column number in the original source that
+  //                       corresponds to this chunk of generated code,
+  //       name: The name of the original symbol which generated this chunk of
+  //             code.
+  //     }
+  //
+  // All properties except for `generatedLine` and `generatedColumn` can be
+  // `null`.
+  //
+  // `_generatedMappings` is ordered by the generated positions.
+  //
+  // `_originalMappings` is ordered by the original positions.
+
+  SourceMapConsumer.prototype.__generatedMappings = null;
+  Object.defineProperty(SourceMapConsumer.prototype, '_generatedMappings', {
+    get: function () {
+      if (!this.__generatedMappings) {
+        this.__generatedMappings = [];
+        this.__originalMappings = [];
+        this._parseMappings(this._mappings, this.sourceRoot);
+      }
+
+      return this.__generatedMappings;
+    }
+  });
+
+  SourceMapConsumer.prototype.__originalMappings = null;
+  Object.defineProperty(SourceMapConsumer.prototype, '_originalMappings', {
+    get: function () {
+      if (!this.__originalMappings) {
+        this.__generatedMappings = [];
+        this.__originalMappings = [];
+        this._parseMappings(this._mappings, this.sourceRoot);
+      }
+
+      return this.__originalMappings;
+    }
+  });
+
   /**
    * Parse the mappings in a string in to a data structure which we can easily
-   * query (an ordered list in this._generatedMappings).
+   * query (the ordered arrays in the `this.__generatedMappings` and
+   * `this.__originalMappings` properties).
    */
   SourceMapConsumer.prototype._parseMappings =
     function SourceMapConsumer_parseMappings(aStr, aSourceRoot) {
@@ -231,14 +263,14 @@ define('source-map/source-map-consumer', ['require', 'exports', 'module' ,  'sou
             }
           }
 
-          this._generatedMappings.push(mapping);
+          this.__generatedMappings.push(mapping);
           if (typeof mapping.originalLine === 'number') {
-            this._originalMappings.push(mapping);
+            this.__originalMappings.push(mapping);
           }
         }
       }
 
-      this._originalMappings.sort(util.compareByOriginalPositions);
+      this.__originalMappings.sort(util.compareByOriginalPositions);
     };
 
   /**