Telling Google what you are looking for can’t be prevented that easy, but it is now possible to prevent telling anybody else who might sit in the middle of your communication.
For some strange reason this toolbar can not be customized via GUI. However the solution is still simple and might be of value for anybody else, thats why I am about to write this
Just enter the directory where you firefox Installation is located and have a look for a file called google.xml inside a subdirectory called searchplugins.
All you need to do here is to replace any apperance of the string http:// by https://
Now restart firefox and enjoy your encrypted search.