Just curious, what is Zotero's business model (as open-source software for academicians)?

It's most direct competition, Mendeley, has always been closed-source, and then got acquired by Elsevier.

To make it clear: I love that Zotero is open-source. And I am happy it is growing (I remember the early versions, well over a decade ago, when I was writing my Master's thesis). I am just curious if they are based mostly on the storage payments, grants, or voluntary work.