The Linux Journal has this advanced tutorial on configuring Firefox. It's rather bloated, but there is some information of note.
The Firefox Web browser, built by the Mozilla Foundation and friends is a complicated piece of technology-if you care to look under the hood. It's not obvious where the hood catch is, because the surface of Firefox (its user interface) is polished up to appeal to ordinary, nontechnical end users. This article gives you a glimpse of the engine. It explains how the Mozilla about:config URL opens up a world of obscure preferences that can be used to tweak the default setup. They're an improbable collection and therein lies the beauty of Firefox if you're a grease monkey or otherwise technical. At the end you'll know a little more about Firefox, but only enough to be dangerous.
Like any Linux-friendly piece of software, Firefox responds to preset environment variables. You can, for example, set the MOZILLA_FIVE_HOME or MOZ_PLUGIN_PATH variables prior to startup. They both work like the standard PATH, so no surprises there. The per-process space available for environment variables is, however, limited, and a simple textual concatenation of attribute-value pairs is a fairly inflexible way to store data. Firefox has a large set of runtime configuration options, and the environment isn't a suitable storage area.
Firefox configuration is stored in a small attribute-type-value database called the preference system. You can see a delta of this data set in the ~/.mozilla/firefox/*/prefs.js file. That file holds only the nondefault values selected by the user. The rest of the preferences either are unstated or stated in install files that are part of the standard install. For me, they're in /local/install/firefox/defaults/pref, because /local is my playpen of choice.
For a technical person, this system is a bit problematic because the full list of preferences doesn't appear anywhere on disk, and the standard way to change those preferences is to use the Firefox User Interface, which also is incomplete. That interface provides GUI elements (buttons, fields and check boxes) for only the most basic of the preferences available. Firefox isn't trying to be Emacs, after all. The rest of the preferences have to be dug up from elsewhere.
That other place is the special string about:config, which can be typed in the Firefox Location bar where the addresses of Web sites are entered. Briefly recall the taxonomy of W3C addresses: URIs (Universal Resource Identifiers) are a special case of IRIs (International Resource Identifiers). A URI either can be a URL (a Uniform Resource Locator) or a URN ( a Uniform Resource Name). It's URLs that we see all the time. They consist of a scheme (typically http), a colon (:) and an address (x.org).
