Firefox Send is part of the Test Pilot program that aims to experiment with some new features before making them a part of Firefox or from Mozilla’s line of products. The Screenshots feature from version 56 was such an experiment that was promoted to an official feature. At the moment there are 6 ongoing experimental features being tested.
Send is an attempt to make file transfer across the internet easy and secure, an alternative to established services like WeTransfer.com. The idea is simple: you drag your file over the page (up to 1GB), the file is encrypted and transferred to the Send server and you get a link you can share with someone. The link is valid for 24 hours or one download.
The service is not limited to Firefox and works in all major browsers.
The team behind the project is requesting your feedback, so if you have some ideas for them hit the Feedback button in the top right corner.