What i did was in one browser tab (Chrome) have my completed project (CodeRoad) showing the tick mark on screen (when you complete all tasks).
Then in a separate tab open the fcc curriculum and load the challenge i just completed and reload the page (i don’t know exactly why this worked).
Once that is done i went back to the other tab (CodeRoad) clicked on the blue button (not sure if it says submit or simply OK) and tried to submit my solution again and it accepted the submission.
In case it does not work with Firefox, give it a try using Chrome.