Mozilla is in the game of keeping the internet healthy. Part of it involves products, such as Firefox and its quest to recover userbase. Parts of it involves money, such as MOSS awarding grants or prizes for FOSS stuff they use and see value.
But most of Mozilla actions can be thought on "how close they get Mozilla to achieve its mission as stated in the Mozilla Manifesto[1]". In the case of Firefox Send, it enables less friction sending files and protects the user privacy, so it IMHO advances item #4 of the manifesto: "Individuals’ security and privacy on the Internet are fundamental and must not be treated as optional." It also serves as a nice branding reminder for people, as it works with other browsers, makes people using some other solution remember Firefox...