The only initial UI - visible just after installing Firebug - is an icon placed at the bottom of the browser window, which I believe is the kind of minimalistic UI users want.
- The user deserves a clear result after the installation - "oh, there is Firebug now".
- The UI is minimal enough so it doesn't disturb the user when Firebug is not used.
(I am going to call this icon Firebug Start Button)
Another important role of this tiny UI is informing the user whether Firebug is currently activated (the icon is shining and colorful) or deactivated (grayed out). This is useful to know since having Firebug activated costs more CPU cycles. Also, don't forget, number of errors is displayed next to it (in case of any errors on the page).
Clicking on the icon opens the real Firebug UI and right clicking on it, opens a context menu with some actions that might be useful at this moment.
So, far so good.