I might misunderstand what you mean by third party service, but today you can connect to any service on the Internet (including databases like Firestore) with Workers using fetch() [1].

We're building out our storage products -- we started with Workers KV, our distributed Key-Value store, and will to continue expanding our offering to support more use cases. You won't be able to build everything with Workers right away, but we've seen customers solve really interesting problems with it. As we add functionality to support more and more use cases, we expect Workers to grow into a really powerful platform.

Note that Cloudflare Workers is a different product from Cloudflare Apps (which is what you see on the Marketplace). Cloudflare Apps does allow developers who want to distribute their Worker to other Cloudflare customers to do so, using Apps with Workers.

[1] More on fetch() here: https://workers.cloudflare.com/docs/reference/runtime/apis/f...