If you get the message when trying to open Firefox or Thunderbird, the reason is that Firefox or Thunderbird cannot write to user´s ~/.cache directory. The solution is to set permissions and ownership for the folder.

sudo chmod 755 -R ~/.cache
sudo chown $USER:$USER -R /home/$USER/.cache

For revealing the owner of files and folders in $HOME directory run

find $HOME ! -user $USER -type f