You can automatically run tests as a part of your build process using TestCafe and Travis CI. There are Linux versions of Chrome and Firefox browsers available for Travis CI builds, so you can run your tests completely in the cloud.
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: