The ff-theme-util is a package you have to uninstall, either in your package manager (Pamac) or in the terminal with
sudo pacman -R ff-theme-util.
Your Firefox profile folder is in your /home/.mozilla/firefox folder called something.default. If you cannot see the .mozilla folder, set your file manager to show hidden folders.
You can also navigate there if you enter
about:support in the address bar in Firefox, then scroll down where it says profile folder and click open.