If your test suite handles a modal dialog popup, for example, a redirect to another location, then you may need to add a custom profile so that the popup is suppressed. This can be fixed by applying a custom Firefox profile with the option turned off: (example is in Ruby using Capybara)
Sure enough, the test in question threw up a dialog, a dialog I had never seen before: “This web page is being redirected to a new location. Would you like to resend the form data you have typed to the new location?” A quick search of this text found a large number of support requests to Mozilla, such as https://support.mozilla.org/en-US/questions/792131 and https://support.mozilla.org/en-US/questions/969705. This dialog comes from Firefox’s Accessibility system, is default to on, and is easily disabled via profile settings. Changing the Firefox profile in Capybara is relatively easy: