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.

Closes gh-4524

.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

@@ -13,8 +13,7 @@ module.exports = function( grunt ) {
 	}
 
 	var fs = require( "fs" ),
-		gzip = require( "gzip-js" ),
-		isTravis = process.env.TRAVIS;
+		gzip = require( "gzip-js" );
 
 	if ( !grunt.option( "filename" ) ) {
 		grunt.option( "filename", "jquery.js" );
@@ -191,9 +190,7 @@ module.exports = function( grunt ) {
 				singleRun: true
 			},
 			main: {
-
-				// The Chrome sandbox doesn't work on Travis.
-				browsers: [ isTravis ? "ChromeHeadlessNoSandbox" : "ChromeHeadless" ]
+				browsers: [ "ChromeHeadless", "FirefoxHeadless" ]
 			},
 
 			jsdom: {