[Trouble Shooting 101] Firefox Alert Is Not Popping Up During Automation

Tips and Tricks
1

Issue
While running automation tests, the expected browser alert does not appear in Firefox, even though it works in other browsers.

Root Cause
In Firefox, the preference dom.disable_beforeunload is set to true by default.
This blocks beforeunload alerts from being displayed.

Solution
You need to explicitly set the Firefox preference dom.disable_beforeunload to false before launching the browser.

Example (Katalon + Selenium):

String browser = DriverFactory.getExecutedBrowser().getName();

if (browser.equalsIgnoreCase(‘FIREFOX_DRIVER’)) {
System.setProperty(“webdriver.gecko.driver”,
DriverFactory.getGeckoDriverPath());

FirefoxOptions firefoxOptions = new FirefoxOptions();
firefoxOptions.addPreference("dom.disable_beforeunload", false);

WebDriver driver = new FirefoxDriver(firefoxOptions);
driver.navigate().to(url);
DriverFactory.changeWebDriver(driver);

} else {
WebUI.openBrowser(url);
}

Important Note
Setting this via Desired Capabilities in Katalon does not work for Firefox, because the FirefoxDriver constructor only accepts FirefoxOptions.

Takeaway
If Firefox alerts are not appearing:

  • Check the dom.disable_beforeunload preference
  • Use FirefoxOptions instead of Desired Capabilities

Hope this helps anyone facing the same issue :+1:

Enjoy testing

2 Likes
2

nice information !

1 Like
3

good one for beginners!!

1 Like
4

just sharing a few additional things that might be worth checking:

  • first, make sure it’s a real browser alert, not a custom modal (SweetAlert, Bootstrap, etc.). Selenium/Katalon can’t handle those with switchTo().alert()
  • check timing issues. In Firefox, alerts can appear very briefly. Using WebUI.waitForAlert() or adding a short explicit wait often helps
  • try running with a fresh Firefox profile. Reused profiles or extensions can sometimes suppress alerts
  • if the alert is triggered after a frame or window change, double-check you’re in the correct frame/window context
  • test the same flow in a minimal script. If it works there, the issue is usually related to the test flow, not the alert itself
  • also worth checking Firefox, GeckoDriver, and Katalon version compatibility, especially if it works in Chrome but not Firefox
1 Like