AltGr triggers email address selection from the contacts autocomplete suggestion list

I'm using Thunderbird 17.0.2 (17.0.2+build1-0ubuntu0.12.04.1) on Ubuntu 12.04.2 (with latest updates) with a French Keyboard, configured with default layout called "Français (variante)".

If I start composing a new message, and type some letters in the recipient address zone, Thunderbird suggests some email addresses from my contacts.
If I don't want to use one of these suggestions, I will have to type the @ symbol at some point.
On a French keyboard, the @ symbol can be obtained with AltGr key and "à" key : you have to press AltGr, then press à, and then release both keys.

The problem is that, as soon as you press AltGr, Thunderbird selects the first email address of his suggestions, and fills the recipient input field with it.
If you don't see it, you end up with a @ symbol at the end of the suggested email address.

The workarounds I found are :
- type quicker than Thunderbird suggests. If you're very fast, it can do the trick
- copy/paste the email address from somewhere else (or at least the @ symbol)

I have this same behavior on 2 different computers : one with Ubuntu x86 and, the other one with x64. Both under Unity 3D with Intel graphics card.
After some searching, I found an old bug on Thunderbird that triggered this exact issue : https://bugzilla.mozilla.org/show_bug.cgi?id=464914 . But it's been fixed in 2009.

It's very annoying because the @ symbol is mandatory in any email address...