Client won't connect to GraphQL /subscriptions endpoint using Sec-WebSocket-Protocol: graphql-transport-ws header

[Teajey]

Subscriptions aren't working for my frontend, but they are working from GraphiQL, so it seems I must have everything set up correctly on the backend. On the frontend I'm using Apollo so it's relatively trivial to get frontend set up correctly too. But when I run the frontend in Firefox I get this message:

Firefox can’t establish a connection to the server at wss://localhost:9443/subscriptions.

The only discernible difference between what GraphiQL sends and what my frontend sends is a header.
What GraphiQL sends:

Sec-WebSocket-Protocol: graphql-ws

And what my frontend sends:

Sec-WebSocket-Protocol: graphql-transport-ws

I've found mention of graphql-ws in the source code here, but not graphql-transport-ws. This doesn't seem to be something that's easily configurable in Apollo, so I'm wondering what the deal is here?

[Teajey]

I've actually just found this note in Apollo's docs.

So it looks like I need to switch to subscriptions-transport-ws until graphql-ws is supported here? #1022