Twistie icons not showing in Firefox 68

When a locally hosted Seascape archive is opened in Firefox 68 or higher, the twistie icons are replaced with a different icon as shown in this screenshot:

The issue is caused due to a change in the way that the origin of the page is defined in Firefox starting with version 68.

More info: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS/Errors/CORSRequestNotHttp To resolve the issue, you'll need to set the privacy.file_unique_origin setting in your Firefox settings to false.

Type about:config in your address bar, look for the privacy.file_unique_origin setting, and double click the Value entry so it changes to false:

After restarting Firefox, the twistie icons will be displayed normally: