Have you checked the permissions on the directory where you installed Mozilla with ls -l? The permissions should look like this if you want all users to have write access to the directory (usually a bad idea, IMO)...
drwxrwxrwx some_user some_group direcotry_name
If users absolutely must have write access, to this directory, the following command (issues as root) will change that directory and all files under it to writable by anyone:
chmod -R a+w <directory name>