Do you have a real physical "azerty" keyboard to connect to your computer? If so you need to find out how to make it work using your operating system's controls. For occasional accented characters there is an insert option in the menu in the Compose window in Thunderbird, allowing you to work with a regular qwerty keyboard. Alternatively, the abcTajpu addon lets you compose áçcëntêd letters (and more!) via your regular keyboard. This works in both Thunderbird and Firefox. And finally, your OS will have its own way to "compose" characters not found on the keyboard. I frequently use alt+0176 to create the ° symbol in technical documents.