chore(karma): re-enable firefox (#3210)

Re-add Firefox to the list of target browsers for wider test coverage
across browsers. 

For now, we target Windows 10 only (like the other browsers). The
goal of this PR is to get programmatic testing under Firefox in place

test/karma/karma.config.js

@@ -12,12 +12,12 @@ const browserStackLaunchers = {
     os: 'Windows',
     os_version: '10',
   },
-  // bs_firefox: {
-  //   base: 'BrowserStack',
-  //   browser: 'firefox',
-  //   os: 'Windows',
-  //   os_version: '10',
-  // },
+  bs_firefox: {
+    base: 'BrowserStack',
+    browser: 'firefox',
+    os: 'Windows',
+    os_version: '10',
+  },
   bs_edge: {
     base: 'BrowserStack',
     browser: 'edge',
@@ -70,6 +70,7 @@ module.exports = function (config) {
     plugins: [
       'karma-chrome-launcher',
       'karma-browserstack-launcher',
+      'karma-firefox-launcher',
       'karma-ie-launcher',
       'karma-edge-launcher',
       'karma-jasmine',

test/karma/package.json

@@ -34,6 +34,7 @@
     "karma-browserstack-launcher": "^1.6.0",
     "karma-chrome-launcher": "^3.1.0",
     "karma-edge-launcher": "^0.4.2",
+    "karma-firefox-launcher": "^2.1.2",
     "karma-ie-launcher": "^1.0.0",
     "karma-jasmine": "^4.0.1",
     "karma-polyfill": "^1.1.0",