diff --git a/browser/devtools/framework/target.js b/browser/devtools/framework/target.js
index 0e5680c4c16e..c268b21f73e5 100644
--- a/browser/devtools/framework/target.js
+++ b/browser/devtools/framework/target.js
@@ -303,6 +303,7 @@ TabTarget.prototype = {
               this._form = tab;
               return true;
             }
+            return false;
           });
           if (!this._form) {
             this._form = aResponse.tabs[aResponse.selected];
diff --git a/browser/devtools/framework/toolbox.js b/browser/devtools/framework/toolbox.js
index 11f0bbe29bfc..f23dd98f4b41 100644
--- a/browser/devtools/framework/toolbox.js
+++ b/browser/devtools/framework/toolbox.js
@@ -35,6 +35,7 @@ loader.lazyGetter(this, "toolboxStrings", () => {
       return bundle.formatStringFromName(name, args, args.length);
     } catch (ex) {
       Services.console.logStringMessage("Error reading '" + name + "'");
+      return null;
     }
   };
 });
@@ -585,6 +586,7 @@ Toolbox.prototype = {
           deck.insertBefore(vbox, deck.childNodes[i]);
           return true;
         }
+        return false;
       });
     }
 
@@ -867,7 +869,7 @@ Toolbox.prototype = {
    */
   switchHost: function(hostType) {
     if (hostType == this._host.type || !this._target.isLocalTab) {
-      return;
+      return null;
     }
 
     let newHost = this._createHost(hostType);
@@ -1016,12 +1018,13 @@ Toolbox.prototype = {
       // This is done after other destruction tasks since it may tear down
       // fronts and the debugger transport which earlier destroy methods may
       // require to complete.
-      if (this._target) {
-        let target = this._target;
-        this._target = null;
-        target.off("close", this.destroy);
-        return target.destroy();
+      if (!this._target) {
+        return null;
       }
+      let target = this._target;
+      this._target = null;
+      target.off("close", this.destroy);
+      return target.destroy();
     }).then(() => {
       this.emit("destroyed");
       // Free _host after the call to destroyed in order to let a chance
diff --git a/browser/devtools/layoutview/view.js b/browser/devtools/layoutview/view.js
index 3d24f7e8d728..532fac0316be 100644
--- a/browser/devtools/layoutview/view.js
+++ b/browser/devtools/layoutview/view.js
@@ -199,7 +199,7 @@ LayoutView.prototype = {
       // If the view is dimmed, no need to do anything more.
       if (this.dimmed) {
         this.inspector.emit("layoutview-updated");
-        return;
+        return null;
       }
 
       for (let i in this.map) {
@@ -247,7 +247,9 @@ LayoutView.prototype = {
       }
 
       this.inspector.emit("layoutview-updated");
+      return null;
     });
+
     this._lastRequest = lastRequest;
     return this._lastRequest;
   }
diff --git a/browser/devtools/sourceeditor/debugger.js b/browser/devtools/sourceeditor/debugger.js
index dcaf5b3494d0..ef76e686cb4f 100644
--- a/browser/devtools/sourceeditor/debugger.js
+++ b/browser/devtools/sourceeditor/debugger.js
@@ -34,8 +34,10 @@ function doSearch(ctx, rev, query) {
   let { cm } = ctx;
   let state = getSearchState(cm);
 
-  if (state.query)
-    return searchNext(ctx, rev);
+  if (state.query) {
+    searchNext(ctx, rev);
+    return;
+  }
 
   cm.operation(function () {
     if (state.query) return;
diff --git a/browser/devtools/sourceeditor/editor.js b/browser/devtools/sourceeditor/editor.js
index 0f68d5231662..d7f4c4d3d20a 100644
--- a/browser/devtools/sourceeditor/editor.js
+++ b/browser/devtools/sourceeditor/editor.js
@@ -145,8 +145,10 @@ function Editor(config) {
 
   // Overwrite default config with user-provided, if needed.
   Object.keys(config).forEach((k) => {
-    if (k != "extraKeys")
-      return this.config[k] = config[k];
+    if (k != "extraKeys") {
+      this.config[k] = config[k];
+      return;
+    }
 
     if (!config.extraKeys)
       return;
@@ -161,11 +163,15 @@ function Editor(config) {
   // indenting with tabs, insert one tab. Otherwise insert N
   // whitespaces where N == indentUnit option.
   this.config.extraKeys.Tab = (cm) => {
-    if (cm.somethingSelected())
-      return void cm.indentSelection("add");
+    if (cm.somethingSelected()) {
+      cm.indentSelection("add");
+      return;
+    }
 
-    if (this.config.indentWithTabs)
-      return void cm.replaceSelection("\t", "end", "+input");
+    if (this.config.indentWithTabs) {
+      cm.replaceSelection("\t", "end", "+input");
+      return;
+    }
 
     var num = cm.getOption("indentUnit");
     if (cm.getCursor().ch !== 0) num -= 1;
@@ -250,8 +256,10 @@ Editor.prototype = {
         let tail = { line: line, ch: this.getText(line).length };
 
         // Shift-click on a gutter selects the whole line.
-        if (ev.shiftKey)
-          return void cm.setSelection(head, tail);
+        if (ev.shiftKey) {
+          cm.setSelection(head, tail);
+          return;
+        }
 
         this.emit("gutterClick", line);
       });
@@ -325,12 +333,15 @@ Editor.prototype = {
   replaceText: function (value, from, to) {
     let cm = editors.get(this);
 
-    if (!from)
-      return void this.setText(value);
+    if (!from) {
+      this.setText(value);
+      return;
+    }
 
     if (!to) {
       let text = cm.getRange({ line: 0, ch: 0 }, from);
-      return void this.setText(text + value);
+      this.setText(text + value);
+      return;
     }
 
     cm.replaceRange(value, from, to);
@@ -757,8 +768,10 @@ Editor.prototype = {
       let cm  = editors.get(this);
       let ctx = { ed: this, cm: cm };
 
-      if (name === "initialize")
-        return void funcs[name](ctx);
+      if (name === "initialize") {
+        funcs[name](ctx);
+        return;
+      }
 
       this[name] = funcs[name].bind(null, ctx);
     });
@@ -901,8 +914,10 @@ function controller(ed) {
         "cmd_findAgain": "findNext"
       };
 
-      if (map[cmd])
-        return void cm.execCommand(map[cmd]);
+      if (map[cmd]) {
+        cm.execCommand(map[cmd]);
+        return;
+      }
 
       if (cmd == "cmd_gotoLine")
         ed.jumpToLine(cm);
diff --git a/toolkit/devtools/css-color.js b/toolkit/devtools/css-color.js
index 71598528a927..4c52738c8f83 100644
--- a/toolkit/devtools/css-color.js
+++ b/toolkit/devtools/css-color.js
@@ -121,9 +121,7 @@ CssColor.prototype = {
   },
 
   get specialValue() {
-    if (SPECIALVALUES.has(this.authored)) {
-      return this.authored;
-    }
+    return SPECIALVALUES.has(this.authored) ? this.authored : null;
   },
 
   get name() {