test: configure Firefox (headless)

Using a local profile directory, this works on modern Linux, too:

karma-runner/karma-firefox-launcher#183 (comment)

.gitignore

@@ -1,5 +1,6 @@
 node_modules/
 dist/
+tmp/
 coverage/
 lib/**/*.d.ts
 .idea/

karma.conf.js

@@ -2,6 +2,8 @@
 
 var path = require('path');
 
+var fs = require('fs');
+
 var coverage = process.env.COVERAGE;
 
 // configures browsers to run test against
@@ -11,6 +13,12 @@ var browsers = (process.env.TEST_BROWSERS || 'ChromeHeadless').split(',');
 // use puppeteer provided Chrome for testing
 process.env.CHROME_BIN = require('puppeteer').executablePath();
 
+var tmpDir = path.join(__dirname, 'tmp');
+
+fs.mkdirSync(tmpDir, { recursive: true });
+
+var firefoxProfile = fs.mkdtempSync(path.join(tmpDir, 'firefox-profile'));
+
 var absoluteBasePath = path.resolve(__dirname);
 
 var suite = coverage ? 'test/coverageBundle.js' : 'test/testBundle.js';
@@ -35,6 +43,14 @@ module.exports = function(karma) {
 
     reporters: [ 'progress' ].concat(coverage ? 'coverage' : []),
 
+    customLaunchers: {
+      'FirefoxHeadless': {
+        base: 'Firefox',
+        flags: [ '-headless' ],
+        profile: firefoxProfile
+      }
+    },
+
     browsers,
 
     browserNoActivityTimeout: 30000,