--- a/browser/tools/mozscreenshots/browser_screenshots.js
+++ b/browser/tools/mozscreenshots/browser_screenshots.js
@@ -4,53 +4,52 @@
 
 "use strict";
 
 /**
  * Finds the index of the first comma that is not enclosed within square brackets.
  * @param {String} env - the string that needs to be searched
  * @return {Integer} index of valid comma or -1 if not found.
  */
-function findComma(env){
-  let ignore = false;
-  for(let i = 0; i < env.length; i++){
-    if(env[i] === '[') 
-      ignore = true;
-    else if(env[i] === ']') 
-      ignore = false;
-    else if(env[i] === ',' && ignore === false){
+function findComma(env) {
+  let nestingDepth = 0;
+  for (let i = 0; i < env.length; i++) {
+    if (env[i] === "[")
+      nestingDepth += 1;
+    else if (env[i] === "]")
+      nestingDepth -= 1;
+    else if (env[i] === "," && nestingDepth === 0) {
       return i;
     }
   }
-  
+
   return -1;
 }
 
 /**
  * Splits the environment variable around commas not enclosed in brackets.
  * @param {String} env - The environment variable
- * @return {String[]} Array of strings containing the configurations 
- * e.g. ["Toolbars[onlyNavBar,allToolbars]","DevTools[jsdebugger,webconsole]","Tabs"] 
+ * @return {String[]} Array of strings containing the configurations
+ * e.g. ["Toolbars[onlyNavBar,allToolbars]","DevTools[jsdebugger,webconsole]","Tabs"]
  */
-function splitEnv(env){
+function splitEnv(env) {
   let result = [];
-  
+
   let commaIndex = findComma(env);
-  while( commaIndex != -1 ){
+  while ( commaIndex != -1 ) {
     result.push( env.slice(0, commaIndex).trim() );
-    env = env.slice(commaIndex+1);
+    env = env.slice(commaIndex + 1);
     commaIndex = findComma(env);
   }
   result.push( env.trim() );
-  
   return result;
 }
 
 add_task(async function capture() {
   let setsEnv = env.get("MOZSCREENSHOTS_SETS");
   if (!setsEnv) {
     ok(true, "MOZSCREENSHOTS_SETS wasn't specified so there's nothing to capture");
     return;
   }
 
-  let sets = splitEnv(setsEnv);
+  let sets = splitEnv(setsEnv.trim());
   await TestRunner.start(sets);
 });

--- a/browser/tools/mozscreenshots/mozscreenshots/extension/TestRunner.jsm
+++ b/browser/tools/mozscreenshots/mozscreenshots/extension/TestRunner.jsm
@@ -126,33 +126,31 @@ this.TestRunner = {
         let match = /\[([^\]]+)\]$/.exec(setName);
         if (!match) {
           throw new Error(`Invalid restrictions in ${setName}`);
         }
         // Trim the restrictions from the set name.
         setName = setName.slice(0, match.index);
         restrictions = match[1].split(",").reduce((set, name) =>
           set.add(name.trim()), new Set());
-		  
       }
       try {
         let imported = {};
         Cu.import("chrome://mozscreenshots/content/configurations/" + setName + ".jsm",
                   imported);
         imported[setName].init(this._libDir);
         let configurationNames = Object.keys(imported[setName].configurations);
         if (!configurationNames.length) {
           throw new Error(setName + " has no configurations for this environment");
         }
-        //Checks to see if nonexistent configuration have been specified
-        if(restrictions != null) {
-          for(let restriction of restrictions){
-            if( !configurationNames.includes(restriction) ){
-              throw new Error(restriction + " is a nonexistent configuration");
-            }
+        // Checks to see if nonexistent configuration have been specified
+        if (restrictions) {
+          let incorrectConfigs = [...restrictions].filter(r => !configurationNames.includes(r) );
+          if (incorrectConfigs.length > 0) {
+            throw new Error("non existent configurations: " + incorrectConfigs);
           }
         }
         let configurations = [];
         for (let config of configurationNames) {
           // Automatically set the name property of the configuration object to
           // its name from the configuration object.
           imported[setName].configurations[config].name = config;
           // Filter restricted configurations.

--- a/npm-shrinkwrap.json
+++ b/npm-shrinkwrap.json
@@ -999,34 +999,34 @@
       "resolved": "https://registry.npmjs.org/slice-ansi/-/slice-ansi-0.0.4.tgz",
       "integrity": "sha1-7b+JA/ZvfOL46v1s7tZeJkyDGzU="
     },
     "sprintf-js": {
       "version": "1.0.3",
       "resolved": "https://registry.npmjs.org/sprintf-js/-/sprintf-js-1.0.3.tgz",
       "integrity": "sha1-BOaSb2YolTVPPdAVIDYzuFcpfiw="
     },
+    "string_decoder": {
+      "version": "1.0.3",
+      "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-1.0.3.tgz",
+      "integrity": "sha512-4AH6Z5fzNNBcH+6XDMfA/BTt87skxqJlO0lAh3Dker5zThcAxG6mKz+iGu308UKoPPQ8Dcqx/4JhujzltRa+hQ==",
+      "requires": {
+        "safe-buffer": "5.1.1"
+      }
+    },
     "string-width": {
       "version": "1.0.2",
       "resolved": "https://registry.npmjs.org/string-width/-/string-width-1.0.2.tgz",
       "integrity": "sha1-EYvfW4zcUaKn5w0hHgfisLmxB9M=",
       "requires": {
         "code-point-at": "1.1.0",
         "is-fullwidth-code-point": "1.0.0",
         "strip-ansi": "3.0.1"
       }
     },
-    "string_decoder": {
-      "version": "1.0.3",
-      "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-1.0.3.tgz",
-      "integrity": "sha512-4AH6Z5fzNNBcH+6XDMfA/BTt87skxqJlO0lAh3Dker5zThcAxG6mKz+iGu308UKoPPQ8Dcqx/4JhujzltRa+hQ==",
-      "requires": {
-        "safe-buffer": "5.1.1"
-      }
-    },
     "strip-ansi": {
       "version": "3.0.1",
       "resolved": "https://registry.npmjs.org/strip-ansi/-/strip-ansi-3.0.1.tgz",
       "integrity": "sha1-ajhfuIU9lS1f8F0Oiq+UJ43GPc8=",
       "requires": {
         "ansi-regex": "2.1.1"
       }
     },