Your browser doesn't support Filesystem Access API.
It will still work, however, you may see very high memory usage when exporting a profile with a large amount of posts and images as it has to collect them all in memory. Consider temporarily switching to Chrome for this.
Note that these data exports aren't useful on its own, this site retrieves your repository as well as your uploaded media files, and stores it as is without any additional processing. It's intended to be consumed by other tools.