Build: Run tests on Travis on FirefoxHeadless as well

Also, run them on both ChromeHeadless & FirefoxHeadless locally on
`grunt karma:main`.

Plus, so far, the chrome addons were installed for all the jobs, even
the ones that weren't used for browser testing. Changing that makes
those jobs faster.

(cherry-picked from 84835e68239ce55d1fc007b284e8ef4ed2817c2)

.travis.yml

@@ -4,14 +4,17 @@ node_js:
 - "8"
 - "10"
 - "12"
-addons:
-  chrome: stable
 env:
   - NPM_SCRIPT=test:browserless
 matrix:
   include:
     # Run browser tests only on one Node.js version to save time.
-    - node_js: "10"
-      env: NPM_SCRIPT=test:browser
+    - node_js: "12"
+      env:
+        - NPM_SCRIPT="test:browser"
+        - BROWSERS="ChromeHeadless,FirefoxHeadless"
+      addons:
+        chrome: stable
+        firefox: latest
 script:
   - npm run $NPM_SCRIPT

Gruntfile.js

@@ -14,7 +14,6 @@ module.exports = function( grunt ) {
 
 	var fs = require( "fs" ),
 		gzip = require( "gzip-js" ),
-		isTravis = process.env.TRAVIS,
 		oldNode = /^v6\./.test( process.version );
 
 	// Support: Node.js <8
@@ -223,9 +222,7 @@ module.exports = function( grunt ) {
 				singleRun: true
 			},
 			main: {
-
-				// The Chrome sandbox doesn't work on Travis.
-				browsers: [ isTravis ? "ChromeHeadlessNoSandbox" : "ChromeHeadless" ]
+				browsers: [ "ChromeHeadless", "FirefoxHeadless" ]
 			},
 
 			jsdom: {