Why do programs like Firefox want to act as a server?

Question

Everytime I load a new program like firefox, it needs to act as a server, a server in trusted zone, acess the internet, and access the internet in a trusted zone. Why are there 4 things, and what do they mean? Especially the server part.

Answer 1

Some browsers like Firefox, Avant Browser etc. have their own error pages, loaded from your computer, like this one:

Oops, we can't open the web page you requested ...
- Error Unknown - Unknown error with URL

This is the reason what they act as a server.