Juzer Burhani

The about:config page in Firefox allows you set preferences and tweak certain settings that are otherwise impossible to edit via the regular Firefox Options dialog. For instance, you can alter the default behavior of address bar, set your own domain guessing rules, change user-agent strings, and so much more.

The only problem is that about:config settings are too technical and packed in a geek interface which can be slightly intimidating to most Firefox users.
Luckily, there’re some excellent add-ins that add a visual interface (GUI) to about:config settings thus enabling anyone and everyone to "hack" Firefox without being a geek.

1. GUI Config - This lets you change the default behavior of mouse clicks, browser cache, download window alerts and a dozen other settings. You can hover the mouse over a setting to know in simple English how that change will affect the browser behavior.
GUI Config is accessible from Tools -> Advanced Settings.

2. Configuration Mania - While GUI Config gives you access to only a limited set of Firefox hidden settings, Configuration Mania supports a more comprehensive list though the help system is not that great.
The extension is nevertheless very powerful and combines features of several add-ins into one. You could change user-agents (to read magazines or WSJ), perform virus-scans after file downloads, disable security checks to install older extensions, etc.
Configuration Mania is accessible from Tools -> Configuration Mania.