What is the actual number of simultaneous connections per domain allowed?

Question

Browsers

According to the documentation, browsers seem to allow no more than 5 - 7 simultaneous connections per domain, but in practice I did not notice problems with either 10 connections or 20 connections. So what is the actual number of connections allowed by browsers?

At the same time, I have two very important questions:
1. Is the limit on the number of connections per domain valid only within one tab or does it apply to the entire browser?
2. Is there a limit on the number of connections inside the extension background process?

Answer 1

6 per domain, 20 per websockets

// Default to allow up to 6 connections per host. Experiment and tuning may
// try other values (greater than 0).  Too large may cause many problems, such
// as home routers blocking the connections!?!?  See http://crbug.com/12066.
//
// WebSocket connections are long-lived, and should be treated differently
// than normal other connections. Use a limit of 255, so the limit for wss will
// be the same as the limit for ws. Also note that Firefox uses a limit of 200.
// See http://crbug.com/486800
int g_max_sockets_per_group[] = {
    6,   // NORMAL_SOCKET_POOL
    255  // WEBSOCKET_SOCKET_POOL
};

Source
The pool manager has methods that allow you to change these values, but they are not used anywhere except in unit tests.

Answer 2

what is the actual number of connections allowed by browsers?

In Firefox-based browsers (Waterfox, Palemoon, etc.) different numbers of connections are controlled via about:config.
Type in a new tab, press Enter and type network.http. As far as I remember, all or at least most of the settings will have names starting in this way.
Some of them are described here: kb.mozillazine.org/Category:Tweaking_preferences
Default values ​​are also indicated there - if any of the parameters is not visible in about:config, it means that it uses the default value, and to change it, the parameter must first be created.