Gotcha.
Unfortunately, the engine that Thread runs on doesn't play well with Firefox or its forks, like Librewolf. It's something I've been trying to fix but haven't been able to resolve yet, and may never be able to fully get working. I recommend either downloading the version that runs on your OS or running the web version in Chrome or a Chromium based browser, like Edge, Vivaldi, Brave, etc.
Sorry!