Hi Folks !
I’m here to suggest a very simple, yet useful implementation to Thunderbird 
Scenario :
• I open my address book to write an email to a specific number of people (I know I have limitations on recipients due to my mail provider)
• I want to select a number of recipients wich wouldn’t exceed the recipients allowed
• And there it is : I can’t see how much recipients I’ve selected !
• I know this is just a request and i’m providing nothing, but maybe there’s Thunderbird devs here, and adding my little gimmick wouldn’t be such a pain
Here you can see what I mean
Another Alternative would be to add a togglable tab in the adress book window to see an index
I hope my proposition is clear, and you may find it worth implementing.
Hit me up if you need more info !
Bye
PS : Bravo for this soft, wich participates resisting for a free web ! 