diff -r 000000000000 -r 6474c204b198 browser/devtools/sourceeditor/codemirror/activeline.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/devtools/sourceeditor/codemirror/activeline.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,66 @@
+// Because sometimes you need to style the cursor's line.
+//
+// Adds an option 'styleActiveLine' which, when enabled, gives the
+// active line's wrapping <div> the CSS class "CodeMirror-activeline",
+// and gives its background <div> the class "CodeMirror-activeline-background".
+
+(function(mod) {
+  if (typeof exports == "object" && typeof module == "object") // CommonJS
+    mod(require("../../lib/codemirror"));
+  else if (typeof define == "function" && define.amd) // AMD
+    define(["../../lib/codemirror"], mod);
+  else // Plain browser env
+    mod(CodeMirror);
+})(function(CodeMirror) {
+  "use strict";
+  var WRAP_CLASS = "CodeMirror-activeline";
+  var BACK_CLASS = "CodeMirror-activeline-background";
+
+  CodeMirror.defineOption("styleActiveLine", false, function(cm, val, old) {
+    var prev = old && old != CodeMirror.Init;
+    if (val && !prev) {
+      cm.state.activeLines = [];
+      updateActiveLines(cm, cm.listSelections());
+      cm.on("beforeSelectionChange", selectionChange);
+    } else if (!val && prev) {
+      cm.off("beforeSelectionChange", selectionChange);
+      clearActiveLines(cm);
+      delete cm.state.activeLines;
+    }
+  });
+
+  function clearActiveLines(cm) {
+    for (var i = 0; i < cm.state.activeLines.length; i++) {
+      cm.removeLineClass(cm.state.activeLines[i], "wrap", WRAP_CLASS);
+      cm.removeLineClass(cm.state.activeLines[i], "background", BACK_CLASS);
+    }
+  }
+
+  function sameArray(a, b) {
+    if (a.length != b.length) return false;
+    for (var i = 0; i < a.length; i++)
+      if (a[i] != b[i]) return false;
+    return true;
+  }
+
+  function updateActiveLines(cm, ranges) {
+    var active = [];
+    for (var i = 0; i < ranges.length; i++) {
+      var line = cm.getLineHandleVisualStart(ranges[i].head.line);
+      if (active[active.length - 1] != line) active.push(line);
+    }
+    if (sameArray(cm.state.activeLines, active)) return;
+    cm.operation(function() {
+      clearActiveLines(cm);
+      for (var i = 0; i < active.length; i++) {
+        cm.addLineClass(active[i], "wrap", WRAP_CLASS);
+        cm.addLineClass(active[i], "background", BACK_CLASS);
+      }
+      cm.state.activeLines = active;
+    });
+  }
+
+  function selectionChange(cm, sel) {
+    updateActiveLines(cm, sel.ranges);
+  }
+});