I can confirm that feedback submitted via Firefox is received from the dashboard.
I have run a test using the newest version of Firefox and I received the confirmation message on both current and future UI.
Therefore you must have a local issue, try the usual, clearing cache, cookies, trying in incognito mode, disable extensions 1 by 1 to see if any are interfering with the process, and possibly reinstall the browser if needed?
Best wishes ✌️