Here is a list of everything available in case anyone might find that more convenient:

Languages:

- C
- C++
- C++ (Firefox)
- Java
- JavaScript
- PHP
- Python
- Rust

C

So you think OOP is for hipsters?
That's cool, we all get nostalgic sometimes. You could work on:

- NSS: The network security pieces of Firefox
https://developer.mozilla.org/en-US/docs/Getting_Started_With_NSS

C++

So you like long compile times and incomprehensible error messages?
That's cool, we do too. You could work on:

- Gecko: The engine that drives Firefox
See: C++ (Firefox)

- Boot2Gecko: The operating system for Android phones built on web technologies
https://wiki.mozilla.org/B2G/Hacking

- Thunderbird: The open source email client
http://developer.mozilla.org/En/Simple_Thunderbird_build

- Seamonkey: The open source web productivity suite
http://www.seamonkey-project.org

- v8monkey: Implementing the v8 API on top of Spidermonkey
http://github.com/zpao/v8monkey

- emscripten: Creating a LLVM-to-JS system to allow porting native code to the web
https://github.com/kripken/emscripten/

- SVG Project: Help with the implementation and testing of Mozilla's
Scalable Vector Graphics engine (https://developer.mozilla.org/en-US/docs/SVG)
https://wiki.mozilla.org/SVG:Contribute

- Windows 8 Integration: The Metro-style enabled desktop browser for Windows 8
https://wiki.mozilla.org/Firefox/Windows_8_Integration
See 3.3 Platform Integration Windows 8 Development

- MathML: Display and represent math formulas on the web
https://wiki.mozilla.org/MathML:Home_Page

C++ (Firefox)

That's cool, but not very specific. You could work on:

- General Firefox work: I don't really care about the details
http://developer.mozilla.org/En/Introduction

- Graphics: Lots of work with OpenGL, DirectX, and Skia making things draw quickly
https://wiki.mozilla.org/Platform/GFX/Contribute

- Accessibility:
Make the web more accessible for users using
assistive technologies (eg. screenreaders)
https://wiki.mozilla.org/Accessibility/Contribute

- Networking: Tcp, http, ssl/tls, spdy, and more
http://wiki.mozilla.org/Networking

- Spidermonkey:
The JavaScript engine: if you like compilers, interpreters, JITs, and more
https://wiki.mozilla.org/JavaScript:New_to_SpiderMonkey

JavaScript

So you're a dynamic individual who thinks that, underneath,
everything is an object? That's cool, we like to dream as well.
You could work on:

- Firefox: Did you know that all of the UI is written in JS?
See: JavaScript (Firefox)

- Mobile Firefox:
It's the Gecko engine, with an Android-specific UI. No Android device required.
https://wiki.mozilla.org/Mobile/Get_Involved

- pdf.js: A PDF viewer written entirely in JavaScript
https://wiki.mozilla.org/PDF.js

- Shumway: a Flash player written entirely in JavaScript
https://github.com/mozilla/shumway/wiki

- The addon SDK: The foundation upon which all new kick-ass addons are built
https://wiki.mozilla.org/Jetpack

- Gaia: the default UI for the web-based mobile operating system Boot2Gecko
https://wiki.mozilla.org/Gaia

- DXR: The intelligent source code indexing system
http://github.com/mozilla/dxr

- Thunderbird: The open source email client
http://developer.mozilla.org/En/Simple_Thunderbird_build

- Seamonkey: The open source web productivity suite
http://www.seamonkey-project.org

- Web development: We have many large, complicated projects that use JavaScript
https://blog.mozilla.com/webdev/get-involved/

- Popcorn: Create interactive media pages that seamlessly integrate video,
audio, and traditional web technologies
http://www.benmoskowitz.com/?p=527

- Windows 8 Integration:
The front end for the Metro-style enabled desktop browser for Windows 8
https://wiki.mozilla.org/Firefox/Windows_8_Integration
See 3.2 Front End Windows 8 Development

- Persona: Implement a new way to safely and easily sign into websites
https://wiki.mozilla.org/Persona

JavaScript (Firefox)

Good choice - there are a lot of different areas if you'd like to specialize.
You could work on:

- Firefox in general - I don't really care:
http://developer.mozilla.org/En/Introduction

- Developer tools:
Implement a suite of tools similar to Firebug directly in the browser
https://wiki.mozilla.org/DevTools/GetInvolved

- Sync: Help users take their personal browsing data on the road
https://wiki.mozilla.org/Services/Sync#Get_Involved

Java

So you're a believer in AbstractMethodFactoryBeans?
That's cool, we all have our vices. You could work on:

- Mobile Firefox:
It's the Gecko engine, with an Android-specific UI. No Android device required.
https://wiki.mozilla.org/Mobile/Get_Involved

- Rhino: It's Spidermonkey in Java. Is this actually still a thing?
https://developer.mozilla.org/en-US/docs/Rhino

PHP

So you like your variable names to include dollar signs?
That's cool, everyone misses Perl once in a while. You can work on:

- WordPress: The code that runs our blogs (http://blog.mozilla.org)
https://wiki.mozilla.org/Webdev/GetInvolved/WordPress

- Marketplace: The PHP client for Marketplace (https://marketplace.firefox.com)
https://github.com/mozilla/Marketplace.PHP

- The Mediawiki-Bugzilla plugin:
The Bugzilla plugin for Mozilla's MediaWiki https://wiki.mozilla.org
https://github.com/mozilla/mediawiki-bugzilla

Python

So you enjoy the paradigm of backtrace-driven development?
That's cool, everyone gets a bit tired of static typing once in a while.
You can work on:

- Firefox: Lots of code is generated by python scripts
http://developer.mozilla.org/En/Introduction

- Web development: We have many large, complicated projects written in Python
https://blog.mozilla.com/webdev/get-involved/

- DXR: The intelligent source code indexing system
http://github.com/mozilla/dxr

- Tools & Automation:
Various projects to facilitate easier automated testing of products
https://wiki.mozilla.org/Auto-tools/New_Contributor

Rust

The new systems programming language created by Mozilla.

- Rust: http://www.rust-lang.org