Removing The 'Live Feed' Button
Removes the ‘Live Feed” button from the address bar

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Tweak Type:	Display
Fx 2:	YES
Fx 3:	YES
Fx 3.5:	YES

Along with the ‘Go’ Button and the Bookmark ‘Star’ (Fx 3) buttons there is also a ‘Live Feed’ button to add clutter to the address bar:

To remove the ‘Live Feed’ Button use ChromEdit and add the following lines to your userChrome.css file:

/* Remove live feed icon in Address url toolbar */
#feed-button {
display: none !important;
}

Once you have added these lines, click Save, then Restart; the ‘Live Feed’ button will be gone when Firefox restarts.

Source