First of all you should know that the mx-underlay class is used in blocking pop-ups.
This is default Mendix, thus is not implemented by the developers.
What might be the case here, is that the Selenium script tries to click somewhere, but the application is still "calculating", thus showing the blocking pop-up.
In that case, try the following code (this will keep the script waiting for 4000 milliseconds):
Another thing you could try is waiting for a specific element to be loaded on the screen (after the action is completed).
More for this, you can find here: https://stackoverflow.com/questions/20009211/getting-selenium-to-pause-for-x-seconds
If all above do not work out for you, I found this thread: https://github.com/mozilla/geckodriver/issues/243
Please try to include "AddAdditionalCapability("specificationLevel", 1);" as shown in this example:
var options = new FirefoxOptions(); options.AddAdditionalCapability("specificationLevel", 1); options.BrowserExecutableLocation = @"C:\Program Files\Nightly\firefox.exe"; using (var driver = new FirefoxDriver(options))
Hope this helps.
Thank you for the reply that really helped.