If you want a different RSS indicator in the address field:
save this image: or this:
and put it into this folder: ~/Library/Application Support/Firefox/Profiles/xxxxxxx.default/chrome/
additionally these code in your userChrome.css

- for the first image:

#feed-button[feeds] {
list-style-image: url("firefox-page-livemarks.png") !important;
-moz-image-region: rect(0px, 16px, 16px, 0px) !important;
}
#feed-button[feeds]:active {
-moz-image-region: rect(16px, 16px, 32px, 0px) !important;
}

- for the second image:

#feed-button[feeds] {
list-style-image: url("safari-page-livemarks.png") !important;
-moz-image-region: rect(0px, 27px, 15px, 0px) !important;
}
#feed-button[feeds]:active {
-moz-image-region: rect(15px, 27px, 30px, 0px) !important;
}

Favicons to the search field:
works with these modifications in your userChrome.css

.searchbar-engine-image {
margin-left: -11px !important;
margin-right: 2px !important;
display: -moz-box !important;
background-color: #fff !important;
}

Favicons to the Bookmarks Toolbar:
works with these modifications in your userChrome.css

.bookmark-item > .toolbarbutton-icon {
padding: 0px 2px 0px 0px !important;
margin: 0px !important;
width: 18px !important;
height: 16px !important;
display: inline !important;
}
.bookmark-item:hover > .toolbarbutton-icon {
background: url("chrome://browser/skin/bookmark-hover-mid.png") repeat-x !important;
}
.bookmark-item:hover:active > .toolbarbutton-icon {
background: url("chrome://browser/skin/bookmark-open-mid.png") repeat-x !important;
}
toolbarbutton.bookmark-item[container]:hover:active > .toolbarbutton-text,
toolbarbutton.bookmark-item[container][open="true"] > .toolbarbutton-text,
toolbarbutton.bookmark-item[container]:hover:active > .toolbarbutton-icon,
toolbarbutton.bookmark-item[container][open="true"] > .toolbarbutton-icon {
background: url("chrome://browser/skin/bookmark-open-mid.png") repeat-x !important;
}

About the userChrome.css:
This file sets the display rules for various elements in the Firefox user interface and is located in the sub-folder called chrome in your profile folder. The file does not exist by default, so you need to create it before you can start adding your preferences. There's actually an example file that exists by default, called userChrome-example.css. Basically, you can just rename that file by removing the -example part.