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

This list is current as of Firefox 3.5.

Extensions

Customization

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.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  * 
   3  * Adjusts font size, underlines URLs, and corrects colors
   4  */
   5 .ac-comment
   6 {
   7   font-size: 100% !important;
   8 }
   9 
  10 .ac-url-text
  11 {
  12   color: #306090 !important;
  13   text-decoration: underline !important;
  14 }
  15 
  16 .ac-url-text[selected="true"]
  17 {
  18   color: white !important;
  19 }
  20 
  21 /* Use monospaced font for AwesomeBar */
  22 #urlbar
  23 {
  24   font-family: monospace !important;
  25   font-size: 14px !important;
  26 }
  27 
  28 /* Combines Stop/Reload buttons, such that only one is shown at a time
  29  *
  30  * Note: May require button reordering (e.g reverse order of stop/reload
  31  * buttons) if the below does not work
  32  *
  33  * Derived from user style: http://userstyles.org/styles/10
  34  * Also, see bug 343396: https://bugzilla.mozilla.org/show_bug.cgi?query_format=specific&order=relevance+desc&bug_status=__open__&id=343396
  35  */
  36 #stop-button[disabled="true"] { display:none; } 
  37 #stop-button:not([disabled]) + #reload-button { display:none; }
  38 
  39 /* Italicize unread tabs
  40  *
  41  * Derived from user style: http://userstyles.org/styles/8247
  42  */
  43 #content tab:not([selected]) { font-style: italic !important; }
  44 
  45 /* Display keyword field in Bookmark dialog
  46  *
  47  * Derived from user style: http://userstyles.org/styles/8250
  48  */
  49 #editBMPanel_keywordRow { visibility: visible; }
  50 
  51 /* Hide search and go buttons */
  52 .search-go-button,
  53 .search-go-button-stack,
  54 #go-button
  55 {
  56   display: none !important;
  57 }