AdBlock is my favorite. I never get annoyed by flashing ads anymore.
My second fave is CompactMenu. With that, my “overhead” window space consists of the Firefox title bar, a single, thin row of buttons/url boxes, and the tab row. CompactMenu replaces the “File/Edit/View/Go…” menu with a single mouse icon, which you then click to see all those choices as sub-menus. It requires an extra mouse click any time you want to access something in the menus, but it lets you fit everything you need in a single row. My row, which consists of images with no text, is: CompactMenu, Back, Forward, Reload, Stop, Print, New Tab, Bookmarks, URL Window, Google Window, and the Throbber.
My “Bookmarks” icon is just a simple folder picture, which I click to open up my bookmarks in a menu. I got it there by creating a folder underneath “Bookmarks Toolbar Folder” with a null name, and put all my bookmarks under there. Then you can just drag that button to the single toolbar without requiring a separate toolbar row.
As for themes, by far my favorite is iCandy Jr.