Karma Firefox Launcher

Installing and Using karma-firefox-launcher

karma-firefox-launcher mirrors the Chrome launcher's API closely, providing Firefox (visible window), FirefoxHeadless (no window), and FirefoxDeveloper (Firefox Developer Edition, if installed) launcher configurations.

npm install --save-dev karma-firefox-launcher

// karma.conf.js
config.set({
  browsers: ['ChromeHeadless', 'FirefoxHeadless']
});

Listing both browsers runs the full suite twice, once per engine, and reports results separately for each.

Common Firefox Launcher Names

Firefox              // visible window
FirefoxHeadless        // no window, standard for CI
FirefoxDeveloper         // Firefox Developer Edition, if installed

• FirefoxHeadless requires a reasonably modern Firefox version with native headless support.
• Karma looks for Firefox in standard install locations automatically before falling back to FIREFOX_BIN.
• Firefox and Chrome occasionally diverge on timing-sensitive or CSS-dependent test assertions — that divergence is often the entire point of testing both.
• Firefox's process startup is sometimes slower than Chrome's; account for this in shared timeout settings.

Firefox Launcher Cheat Sheet

Configuration snippets for common Firefox launcher needs.

Custom Firefox Preferences via customLaunchers

karma-firefox-launcher supports a prefs object on custom launcher configurations, letting you tweak Firefox profile preferences — for example, disabling animations that can otherwise interfere with timing-sensitive UI tests.

customLaunchers: {
  FirefoxHeadlessNoAnim: {
    base: 'FirefoxHeadless',
    prefs: {
      'toolkit.cosmeticAnimations.enabled': false
    }
  }
},
browsers: ['FirefoxHeadlessNoAnim']

Troubleshooting FIREFOX_BIN

Just like Chrome, when Firefox isn't installed in a location the launcher checks automatically (common on minimal Linux CI images), setting FIREFOX_BIN explicitly resolves the launch failure.

export FIREFOX_BIN=/usr/lib/firefox-esr/firefox-esr
karma start --single-run --browsers FirefoxHeadless

Common Mistakes

• Assuming Firefox and Chrome will always produce identical results for CSS or timing-sensitive tests.
• Not installing Firefox at all on a CI image before configuring FirefoxHeadless in the browsers list.
• Running Firefox and Chrome with identical shared timeouts, without accounting for Firefox's sometimes slower startup.
• Overlooking FIREFOX_BIN as the fix for a launch failure that looks unrelated to configuration at first glance.

Key Takeaways

• karma-firefox-launcher provides Firefox, FirefoxHeadless, and FirefoxDeveloper launcher configurations.
• Adding Firefox alongside Chrome catches genuine cross-browser discrepancies that single-browser testing misses.
• FIREFOX_BIN is the standard fix when Firefox isn't found in an expected install location.
• Custom launcher prefs objects let you tune Firefox profile settings without a full custom plugin.