"Ever since its debut, Firefox has garnered a reputation for
being an enormously customizable program, both through its add-on
architecture and its internal settings. In fact, many of Firefox's
settings aren't exposed through the Tools > Options menu; the
only way to change them is to edit them manually. In this article,
we'll explore some of the most useful Firefox settings that you can
change on your own, and that aren't normally available through the
program's graphical interface.
"The closest analogy to how Firefox manages its internal
settings is the Windows Registry. Each setting, or preference, is
given a name and stored as a string (text), integer (number) or
Boolean (true/false) value. However, Firefox doesn't keep its
settings in the registry, but in a file called prefs.js. You can
edit prefs.js directly, but it's often easier to change the
settings through the browser window..."