Comment by plaguuuuuu

Mozilla isn't sending that data. You're sending that data.

These terms are common on webapps because, well, they're webapps. You send your data to the webapp and they store it on their own servers.

Web apps, operated by individuals or organisations, are very different to local apps which are operated by you. Just because the Firefox app is running on your device doesn't mean that Mozilla is operating it.

By granting Mozilla the right to access and use your data, you're agreeing to give them data which they never had previously - instead of just sending the POST to xyz.com you're now sending it to Mozilla as well who can do whatever they like with it, sell it to ad networks, whatever.