One of the largest reasons I use Mozilla Firefox is its huge universe of extensions, scripts, and other add-ons to it behave the way you want it to. I've customized my Firefox installation pretty extensively.

This list is current as of Firefox 3.0.

Extensions

Web development

Other stuff

Preferences

user.js settings I use:

Setting

Value

Rationale

layout.css.dpi

0

layout.spellcheckDefault

2

ui.caretBlinkTime

0

ui.allow_platform_file_picker

false

network.http.pipelining

true

network.http.pipelining.ssl

true

network.http.proxy.pipelining

true

network.http.pipelining.maxrequests

16

network.protocol-handler.external.mailto

true

network.proxy.socks_remote_dns

true

nglayout.initialpaint.delay

100

browser.ssl_override_behavior

2

Turn off annoying SSL certificate behavior

browser.xul.error_pages.expert_bad_cert

true

browser.backspace_action

1

Chrome

Contents of my userChrome.css:

   1 /* Font preferences for AwesomeBar dropdown */
   2 .ac-comment {
   3   font-size: 100% !important;
   4 }
   5 
   6 .ac-url-text {
   7   color: #306090 !important;
   8   text-decoration: underline !important;
   9 }
  10 
  11 .ac-url-text[selected="true"] {
  12   color: white !important;
  13 }
  14 
  15 /* Monospaced font for AwesomeBar */
  16 #urlbar {
  17   font-family: Inconsolata !important;
  18   font-size: 17px !important;
  19 }