Unable to click addon on Firefox

Screen Shot
[https://imgur.com/XvRpLn3]
[https://imgur.com/Zpdl0Go]

I have a scenario where i go to simple-modify-header addon and then click on configure button. In windows os everything is working fine, but in Linux I am not able to get the pop-up displayed, after I click on addon.

The following code is used to navigate to addon

           //move to toolbar
    Robot robot = new Robot();
    robot.keyPress(KeyEvent.VK_CONTROL);
    robot.keyPress(KeyEvent.VK_L);
    robot.keyRelease(KeyEvent.VK_L);
    robot.keyRelease(KeyEvent.VK_CONTROL);
    Thread.sleep(2000);

    //move to addons
    robot.keyPress(KeyEvent.VK_TAB);
    robot.keyRelease(KeyEvent.VK_TAB);
    Thread.sleep(1000);
    robot.keyPress(KeyEvent.VK_TAB);
    robot.keyRelease(KeyEvent.VK_TAB);
    Thread.sleep(1000);

    //move to simple-modify-header addon
    robot.keyPress(KeyEvent.VK_RIGHT);
    robot.keyRelease(KeyEvent.VK_RIGHT);
    Thread.sleep(2000);
    robot.keyPress(KeyEvent.VK_RIGHT);
    robot.keyRelease(KeyEvent.VK_RIGHT);
    Thread.sleep(2000);
    robot.keyPress(KeyEvent.VK_RIGHT);
    robot.keyRelease(KeyEvent.VK_RIGHT);
    Thread.sleep(2000);


    //click simple-modify-header addon
    robot.keyPress(KeyEvent.VK_ENTER);
    robot.delay(2000);
    robot.keyRelease(KeyEvent.VK_ENTER);
    Thread.sleep(1000);
    Thread.sleep(1000);

//click on configure addon
    robot.keyPress(KeyEvent.VK_TAB);
    robot.keyRelease(KeyEvent.VK_TAB);
    robot.delay(2000);
    robot.keyPress(KeyEvent.VK_TAB);
    robot.keyRelease(KeyEvent.VK_TAB);
    robot.delay(1000);
    robot.keyPress(KeyEvent.VK_ENTER);
    robot.keyRelease(KeyEvent.VK_ENTER);
    Thread.sleep(3000);