The permission was 755 because I've tried change to 777 first, and before to 755. I created a file inside /mnt/sda1, changed permission, but without success. Before I changed partition format to FAT32, no success. So I plugged another pendrive, and I can save inside it. I don't know why Firefox does not write to internal hd.
ls -l /etc/mtab
lrwxrwxrwx 1 root root 12 Sep 25 04:22 /etc/mtab -> /proc/mounts