Ok, I’ve fixed that error, Firefox shouldn’t break on this anymore. Update with Joomla updater.
And let me explain a bit more how it works:
Assigning character on the backend will not be shown on the fronted for users. So if a user set his/her own characters in profile changes the character ONLY if the character is not assigned to any other players. And if you change a character assignment on the backend it will not change the profile settings of the user. It’s normal, maybe I’ll change it in the future, but now its normal.