The Lazy Dev

Building mozilla is all about having the right environment set up and then follow these simple instructions:
https://developer.mozilla.org/En/Prism/Build

Very short page with few steps from source checout to the actual build. Nice.

So complexity is all about prerequisites. Let's see what these prerequisites are in different Operative system

Windows:
Let's start with this long page: https://developer.mozilla.org/En/Developer_Guide/Build_Instructions/Windows_Prerequisites
From there we need to manually download and install: a Visual Studio, a Windows SDK, a Mozilla Build package and configure something else about ATL studying this page https://developer.mozilla.org/en/atlbase.h , taking care to a eight bullet points long paragraph about "Common problems, hints and restrictions". It seems lot of work and thousand of possible things that could not work or have problems.
Scary.

Linux:
In a terminal:

<pre>apt-get build-dep firefox
apt-get install mercurial libasound2-dev libcurl4-openssl-dev libnotify-dev libxt-dev libiw-dev mesa-common-dev autoconf2.13
 </pre>

Finished.

Make your choice.