If you prefer the icons used before Adblock Plus 0.7.2 (the cockroach), you can use this user style to restore it (best to be used with Stylish). But if you have a better icon, you can make your own style using this template:
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
@-moz-document url("chrome://browser/content/browser.xul"),
url("chrome://navigator/content/navigator.xul"),
url("chrome://messenger/content/messenger.xul"),
url("chrome://messenger/content/messageWindow.xul"),
url("chrome://midbrowser/content/midbrowser.xul"),
url("chrome://emusic/content/startup.xul"),
url("chrome://webrunner/content/webrunner.xul"),
url("chrome://global/content/customizeToolbar.xul") {
toolbar[iconsize="small"] #abp-toolbarbutton,
#PersonalToolbar #abp-toolbarbutton,
#abp-status {
list-style-image: url("file:///c:/icons/abp-16x16.png") !important;
}
#abp-toolbarbutton {
list-style-image: url("file:///c:/icons/abp-24x24.png") !important;
}
}
Here the images (one with three 16×16 icons and another with three 24×24 icons stacked vertically) are assumed to be located in the directory c:\icons, so you might need to change the addresses. To apply this style you can either use Stylish or edit the file chrome/userChrome.css in your profile directory (where is my profile directory?) and add this code. If userChrome.css doesn't exist yet you have to create it.
Note: The changes will take effect after you restart your browser.