Newbie HOWTO: use "about:config" to set this key/values.
Set them to TRUE if you want Ctrl+Left/Right-Arrows stop at the start/end of alpha-numeric words (like what you get on Gtk+/Gnome). Default values make it to jump to the next/previous Space/Tab character! The second one also affects word boundary on selecting with mouse double-click.
(Mozilla >= 1.9, Firefox >= 3.0a1)
This option allows you to select whether triple click should select the whole paragraph, or just the line you are clicking. If it's FALSE, you can select the whole paragraph with quadruple-click (Wait, it doesn't work on Gtk+/Gnome, as Gtk+ doesn't support quadruple-click. If you like it, here are the Mozilla bug: #348751.)
If you use Tor, or any other proxy in your LAN, just set the first one to a big number (I use 50). Also I set the second one to 8, as Flickr and many other photo-sharing sites use just one domain for almost all images on the their pages, so I cannot get even two pages simultaneously. (maps.google.com is smart enough)
Here are how you can define a protocol and set the external application. In this example, I set ED2K protocol. Create the first key as Boolean, and set the value to TRUE. Then create the second key as String, and set the path to your ed2k link-handler ("/usr/bin/ed2k" for me) as its value. That's it.
Homework: Create a protocol-handler and write a script to handle Yahoo! Messenger links, and make GAIM do the requested action (add buddy, send message, etc). Of course you should mail them to me to get your point. :D)
Try possible values for this key (0..4 IIRC) to get more functions under your fingers (horizontal scroll of your professional mouse, or the touch-pad of your laptop).
If you live in a country (or organization) which the default paper size is A4, not Letter, just set the value of this key to "A4".
And if your want to set the default ".com" value to something else (i.e. ".co.uk" or ".ir"), just set it in this key.
And this is the best part for Arabic, Iranian (Persian), and Israeli (Hebrew) users. Setting this key to TRUE will do some magic for you. You can switch the text direction and alignment of input fields with just one keystroke: Ctrk+Shift+X! Also you can switch the page direction from the View menu. Help yourself!
If you use Mozilla/Firefox in a right-to-left locale, just set the value of these keys to "1" instead of default "-1". Here are the Mozilla bug: #350594.
Ok, that's all. Let me know if you have problem with this hacks, or other L10n (localization), Bidi (bi-directional), or RTL (right-to-left) problems with Mozilla/Firefox.