As I had already told you that Mozilla has taken design of Firefox 4 from Chrome and Opera, the orange button which you see in Firefox at the top left corner which contains all menu is first introduced by Opera team.
If you too feel that button is same in Opera and Firefox and wanna make some difference in both or in case you just wanna customize your Firefox then you may use Iconic Firefox Menu, it’s a nice Add-on for Firefox,
It will shrink and and make that orange button into a transparent one, see below screenshot :-
How it, nice na? You don’t have to do anything, just install add-on from below link and restart Firefox :-