Along with all the great and not so great extensions and themes you can install to customise Firefox - you can also customise it even further via the userChrome.css. This where you can place those little tweaks to improve Firefoxs functionality just that little bit more. There are loads of these tweaks out there, just do a google search for Firefox userchrome.css
The userChrome.css file is where you place all of your customizations. It is located inside the "chrome" folder under your user profile. This file can be edited with any plain text editor like EditPad, Notepad, Kate, or Kwrite to name a few. Do NOT use Microsoft Word, OpenOffice.org Writer, or any similar document program. userChrome.css MUST be saved as plain text.
The simplist way to maintain your userChrome.css file is with the ChromEdit extension. It will automatically find the proper location to store this file, and you can maintain the file's contents from a toolbar button or by selecting "Edit User Files" from the Tools menu.
How to add your entries:
NOTE: In your Chrome directory you will find a file called userChrome-example.css. Use this file as your starting point. When finished editing, rename this file to: userChrome.css.
1. If your userChrome.css file is empty, you need to first add this or nothing you place into it will work.
Copy and paste this entire entry into userChrome.css:
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
2. Now you are ready to begin adding your userChrome options. A simple rule to follow is this:
* If the statement begins with @import, the entry is always placed ABOVE the @namespace entry.
* All other entries are placed BELOW the @namespace entry.
3. You need to restart the program before any changes made to userChrome.css will take effect.
Good Luck
Here are some of mine.
NOTE: If you wish to place an icon, as in my first tweak, that image must be in the same directory as your userChrome.css with the size no larger than 48 x 48 pixels.
/* Add this menuitem image for FoxClocks Extension. */
menuitem[label="FoxClocks"] {
list-style-image: url("foxclocks.png") !important;
}
/* Change throbber image. */
#navigator-throbber,
toolbar[iconsize="small"] #navigator-throbber,
toolbar[mode="text"] #navigator-throbber {
list-style-image: url("throbber-still-image.gif") !important;
}
#navigator-throbber[busy="true"],
toolbar[iconsize="small"] #navigator-throbber[busy="true"],
toolbar[mode="text"] #navigator-throbber[busy="true"] {
list-style-image: url("throbber-animated-image.gif") !important;
}
/* Turn Location Bar Green for HTTPS */
#urlbar[level="high"] .textbox-input-box
{ background-color: #33FF33 !important; }
/* Activate up & down movement on personal toolbar bookmark items. */
toolbaritem toolbarbutton.bookmark-item:hover:active,
toolbaritem toolbarbutton.bookmark-item[open="true"] {
padding: 3px 2px 1px 4px !important;
}
/* Change the appearance of toolbar bookmark hover text.
* (Use any color of your liking.) */
toolbarbutton.bookmark-item:hover .toolbarbutton-text {
background-image: none !important;
background-color: #FFFF66 !important;
color: #FF0000 !important;
}
/* Display images for disabled menuitems. */
menu[disabled="true"] > hbox > image,
menuitem[disabled="true"] > hbox > image {
visibility: visible !important;
}
/*Display dotted line around unread Tabs*/
#content tab:not([selected="true"]) {
border-style: dotted !important;
}
/* Change all tab icons to 100% opacity. */
.tab-icon-image {
-moz-opacity: 1.0 !important;
}