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Conceptual Architecture of
Firefox 6.0
By Fully Optimized eXperience (FOX)
Rob Staalduinen 06009513
Katie Tanner 06060742
Gordon Krull 06003108
James Brereton 06069736
What is Firefox?
No, but really…
http://xkcd.com/198/
What is Firefox?
 A free, open source web browser
 Developed and managed by the Mozilla corporation
 Has approximately 25% of worldwide usage share
of web browsers as of September 2011
 Moved to a rapid release cycle in 2011 releasing a
new version every 1 to 2 months
Research Overview
 Began by reading reference architecture for web
browsers provided in the paper “A Case Study in
Architectural Analysis: The Evolution of the Modern
Web Browser” by Alan Grosskurth and Michael W.
Godfrey
 We did general research on Firefox as a whole before
isolating different subsystems
 Then did in-depth research on each subsystem and
began mapping out the architecture
 Important sources of Documentation
 Mozilla Developers Network (MDN)
 Mozilla Wiki
Conceptual Architecture
 Initially, we were expecting a layered architecture
for Firefox
 Within the layers, we expected that the distinct
components were implemented in an object
oriented architectural style
 We were surprised to see that many of the elements
of the lower layers were combined with the upper
layer
Conceptual Architecture
with Reference Architecture
Image source: Figure 3 from “A Case Study
in Architectural Analysis: The Evolution of
the Modern Web Browser” by Alan
Grosskurth and Michael W. Godfrey
XUL & XULRunner
 XUL = XML UI Language
 Firefox’s user interface is not hardwired into the application,
but loaded from a separate UI description. XUL is the
language in which these UI descriptions are built.
 Based on XML (extensible markup language)
 Used to communicate data between applications
 XULRunner
Runtime package that can be used to bootstrap XUL &
XPCOM applications such as Firefox
 Its goal is to facilitate development and deployment of
internet applications
 Includes mechanisms for installing, upgrading, and
uninstalling Firefox

Data Persistence
 Session Store API
 Makes it possible for extensions to easily save and restore
data across Firefox sessions
 Key scenario in which supporting this feature can be crucial
for an extension: Firefox 2 and up lets users undo the
closing of tabs
 Implemented using the nsISessionStore interface
 nsISessionStore: provides a means for extensions and
other code to store data in associations with browser
sessions, tabs and windows
 nsISessionStartup: handles session restore process
Data Persistence
 Storage (mozStorage)
 Is a SQLite database API
 It is available to trusted callers, meaning extensions and
Firefox components only
 DOM Storage
 Designed to provide larger, securer, and easier-to-use
alternative to storing information in cookies
 A means through which string key/value pairs can be
securely stored and later retrieved for use
 Useful because there are no good browser-only methods
exist for persisting reasonable amounts of data for any
period of time
 Allows for “working offline” for extended periods of time
Gecko
 Primarily it is the Browser Engine and Rendering Engine
 Gecko lies at the heart of the Firefox Conceptual Architecture
 Uses an Object-Orientated Architecture to implement the
different components
 Has support for many different standards
 Through evolution from previous versions, Gecko now
contains more than just the tools to render pages but also
contains things such as:
 Networking and Security Libraries
 JavaScript Interpreter
 XML Parser (Expat) is now a library in the HTML Parser
Gecko Conceptual
Architecture
Gecko Components
 Document Parser (HTML & XML Parser)

Parses both the HTML code and XML code and passes it on to the
Content Model for further use
 Style System

The Style System contains the CSS Parser and is responsible for
getting the CSS data from Necko and parsing it before sending it to
the frame constructor
 Platform-Specific Rendering and Widgets

Acts as a Library for the frame constructor to call upon specifics of the
platform the browser is running on using the GTK+/X11 libraries
before passing it on to the UI
 Image Library

Interacts with Necko in order to retrieve image data before sending it
to the Frame Constructor
Gecko Components
 Content Model
 Interacts with the various components of Gecko, DOM
Storage to gather all the data needed before sending it
to the frame constructor
 Frame Constructor
 Carries out the task of piece together all the
information and actually from the rendered web page
before sending it back to the UI through the PlatformSpecific Rendering subsystem
SpiderMonkey
 SpiderMonkey, contained within Gecko, is Firefox’s
JavaScript engine, written in C/C++
 Its components are a compiler, interpreter, 2 just-intime (JIT) compilers, a decompiler, garbage
collection, and a standard library
 Also contains a few public APIs
Necko
 Necko is the main networking library for Mozilla
Firefox
 This library is platform independent, allowing
Firefox to be versatile, true to its open source nature
 Necko itself is split into a layered architecture, with
a pipe-and-filter style flow of execution
 It relies heavily on the XPCOM library, and uses the
NSS library for its network security
Necko Conceptual
Architecture
NSS & PSM
 NSS
 Network Security Services
 Set of libraries, APIs, utilities and documentation
designed to support cross-platform development of
security-enabled client and server applications
 PSM
 Personal Security Manager
 Consists of a set of libraries that perform
cryptographic operations on behalf of a client
application
 Built on top of NSS
XPCOM
 XPCOM = Cross-Platform Component Object Module
 Breaks up large software projects into modular
components, which are reassembled at runtime
 Provides tools and libraries that enable the loading and
manipulation of these components
 XPCOM provides the means of accessing the Gecko
library functionality and embedding or extending Gecko
 This modular programming benefits the reuse, updates,
performance, and maintenance of software
GTK+/X11 Libraries
 GTK+ and X11 are used to create Graphical User
Interfaces (GUIs) on multiple platforms
 Form the Display Backend of the Architecture
 They are used by Gecko (specifically the Frame
Constructor) to provide platform specific data
 The User Interface (XUL) calls upon the libraries
along with Gecko to display the page in the format
for each platform
Rendering a cached webpage
(Sequence Diagram)
UI
Gecko
Necko
SpiderMonkey
Display Backend
Check for cached page
(found)
Send page to Javascript interpreter
Send page to Display Backend to
be rendered
Data Persistence
Summary
 Firefox overall is implemented in a layered architectural
style with Gecko containing most of the subsystems
 Gecko uses an Object Oriented Architecture to organize
and implement the components needed to render web
pages
 Necko is now within Gecko along with the security
protocols it relies on.
 The documentation was largely out-of-date or
incomplete which made extracting the architecture
difficult, due to the nature of open-source projects
Resources

https://developer.mozilla.org/en/Mozilla_Application_Framework_in_Detail

https://developer.mozilla.org/en/NSS_FAQ

https://developer.mozilla.org/en/Necko

http://www.mozilla.org/projects/security/pki/psm/

https://developer.mozilla.org/en/Necko_Architecture

https://wiki.mozilla.org/Gecko:Home_Page

http://www.w3counter.com/globalstats.php

https://wiki.mozilla.org/Gecko:Overview

https://developer.mozilla.org/en/Creating_XPCOM_Components/An_Overview_ 
of_XPCOM

https://developer.mozilla.org/En/SpiderMonkey/Internal

https://developer.mozilla.org/en/Gecko_FAQ

http://research.cs.queensu.ca/home/emads/teaching/readings/emsebrowserRefArch.pdf
https://developer.mozilla.org/en/DOM/Storage

http://www.gtk.org/

https://developer.mozilla.org/en/Session_store_API

https://developer.mozilla.org/en/Gecko_Embedding_Basics

https://developer.mozilla.org/en/XPCOM_Interface_Reference/nsISessionStore

http://xkcd.com/198/

https://developer.mozilla.org/en/XPCOM_Interface_Reference/nsISessionStartup 
https://wiki.mozilla.org/Firefox/Roadmap

https://developer.mozilla.org/en/XUL_Tutorial/Persistent_Data
http://www.x.org/wiki/

https://developer.mozilla.org/en/NSS

http://www.mozilla.org/projects/security/pki/nss/

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