Steps to reproduce:
I've opened 20 tabs on the same site (same domain) and didn't close them. This web site use EventSource on each page.
The first 15 tabs are opened correctly. The 5 next show a blank page with 'Connecting...' as a title and a loader. And nothing else happened until I closed one tab, not even a timeout after several minutes.
The browser used one connection for each EventSource on the 15 first tabs and hit a limit. If I increase network.http.max-connections-per-server in about:config, I don't reproduce the problem.
At least, having an error message, not just a blank page.
Ideally, the browser should have not allowed the EventSource to take all the connections and could load the page.