Necko Lingo

Words We Keep Throwing Around Like Internet Confetti!

Background Thread

Any thread that is not main thread.
Or this thread created here for PBackground for IPC
Usually threads either have a dedicated name but background threads might also refer to the background thread pool: NS_DispatchBackgroundTask.


See nsIChannel.idl. It usually means nsHttpChannel.

Child Process

Usually a firefox forked process - not the main process.
See GeckoProcessTypes.h for all process types in gecko.

Content Process

Usually a firefox forked process running untrusted web content.


Refer RFC 8484 - DNS Queries over HTTPS (DoH).
Resolves DNS names by using a HTTPS server.
Refer this link for more details.


Web API for server-sent-events.
Refer MDN documentation for more details.
Necko is responsible for maintaining part of this code along with DOM.


Also known as E10S (E + 10 chars + S).
The process to make web content run in its own process.
Extended by the ‘Fission’ project, which introduced isolation for sites (really eTLD+1’s).
Refer wiki page for more details.


Fetch standard aims specifies standard for fetching resources across web.
Fetch and Fetch API slightly different things.

Fetch API

Web API for fetching resources from the web. The code is jointly maintained by DOM team and Necko team


Similar to Electrolysis, but different domains (eTLD+1’s) get their own content process to avoid Spectre attacks.
Max 4 processes per eTLD+1.
Iframes get isolated from the parent.
Also referred to as origin isolation.


HTTP version: 0.9 / 1.0 / 1.1 / 2 / 3.

Happy Eyeballs

RFC 6555/8305 – connecting via IPv4 and IPv6 simultaneously.
We implement this in a different way.


HTTP Strict Transport Security.
HSTS preload - a list of websites that will be upgraded to HTTPS without first needing a response.


Object containing information about about the load (who triggered the load, in which context, etc). Refer nsILoadContext for more details.


In async programming, we usually trigger an action, and set a listener to receive the result.
In necko it’s most usually referring to the listener of a channel, which is an object implementing nsIStreamListener and/or nsIRequestObserver.


Every process has a MainThread, which is also the master event loop for the process.
Many things are scoped to run on MainThread (DispatchToMainThread, etc).
For content processes, MainThread is where normal JS content runs (Worker JavaScript runs on DOM Worker threads).


Browser tests, run with full UI.
Refer this for more details.

neqo v/s Necko

neqo is the name of the Mozilla QUIC stack. Sometimes pronounced “knee-ko” or “neck-ou”, “knee-q-oo”.

Necko is the project name of the Mozilla networking stack.


An acronym for the Network Security Services. Refer this for more details.


An acronym for Netscape Portable Runtime Necko’s scope?


Terminology used for referring the classes implementing the Observer design pattern.
Refer the following interfaces for more details:


The PSM acronym may also be described as “Platform Security Module”.
Glue code between Gecko and NSS.
Refer this for more details.

Parent Process

The process that runs the main structure of the browser, including the UI. It spawns the other processes needed for the browser.
Generally it is unrestricted, while most other processes have some level of sandboxing of permissions applied.
Before e10s, all code ran in the Parent Process.


Abstraction encapsulating security details of a web page. Refer the following links for more details:


Abbreviation for Off Main Thread. OMT refers to processing data in non-main thread. There has been efforts in the past to move the processing of data to non-main thread to free up main thread resources.


  • OnStartRequest is listener notification sent when the necko has parsed the status and the header.

  • OnDataAvailable is a listener notification sent when necko has received the the data/body.

  • OnStopRequest is a listener notification sent when necko has received the complete response.

  • Refer to the following interface documentation for more details:


An IETF transport protocol RFC9000 primarily designed to carry HTTP/3, but now also used as a general-purpose Internet transport protocol for other workloads. Implemented in RUST and maintained by neqo.


Race cache with network. Feature that will send a request to network and cache at the same time and take the first to resolve.

Socket Process

WIP project to move the actions of the socket thread into its own process for the purposes of isolation for security and stability (during crashes).

Socket Thread

From the main process, a thread that handles opening and reading from the sockets for network communication.
We also use socket thread in content process for PHttpBackgroundChannel.ipdl


Necko supports SOCKS proxy.
Refer RFC 1928 for more details.


Transport Layer Security. It’s implementation is maintained by NSS team.


Team process of bug intake, analysis and categorization.


Trusted recursive resolver.This is the name of our DoH implementation, as well as the name of the program that ensures DoH providers included in Firefox have agreed not to spy on users.
Refer the following for more details:


Server/client connections over TCP to pass data. A replacement for long-poll HTTP connection.
Refer RFC 6455 and whatwg spec.


A mechanism similar to WebSockets to transfer data between server and client, but built for HTTP/3; can also run over HTTP/2 (being implemented in gecko).


Unit tests for testing our XPCOM code from JS context. Runs without a UI in a simpler setup (typically single-process).
Refer firefox-source-docs for detailed explanation.


XMLHttpRequest. One way to perform AJAX, essentially a way to dynamically make network requests for use in the callers webpage, similar to Fetch.