Gecko:Getting Started
The first step is to [download and build](https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/Build_Instructions) Mozilla or Firefox.
See the information on hacking Mozilla for how to work with the Mozilla project and coding conventions.
If you get stuck at any point, send a question to a Mozilla newsgroup, or get onto IRC at irc.mozilla.org, channel #developers. Do not give up, there are people who want to help you.
If you're looking for a "first bug" to get started with, check out Gecko:First Bugs.