For this click start> Run> type-

%appdata%\Mozilla\firefox\Profiles

open the folder you see, now open another folder name 'gm_scripts'

open 'gmail_yahoo_msn_hotmail_' folder

open a files inside that in notepad

find these lines ( should be line number 17)

//Add your user ID's to this array in the format bellow...
var names = ['youmailid1','youmailid2'];

and change to
//Add your user ID's to this array in the format bellow...
var names = ['you@yahoo.com','you@gmail.com'];

Now you are done...

you can check one example here

Enjoy

NOTE: FIREFOX VERSION 3 USERS ARE REQUESTED TO UNINSTALL AND THEN REINSTALL THIS SCRIPT...