Firefox limits the persistent connections that the browser can have open to a single server. For most use cases, this will not be a problem. But a user who opens multiple Shiny Docs with many embedded Shiny components all at once may hit this limit.
To resolve this problem, increase the network.http.max-persistent-connections-per-server setting (we tested by setting it to 50). Once that limit is increased, you should be able to open many complex Shiny Docs simultaneously from Firefox without issue.