Right now we have the earlybird warning page: https://start.thunderbird.net/en-US/earlybird/

It's not localized, and we should probably change the copy to not talk about Earlybird:

"This version is no longer a supported Thunderbird release and does not receive security updates!"
"We strongly recommend downloading the latest stable version of Thunderbird."

After localization, we should redirect start pages to it. I suggest everything below Thunderbird 52(So 45, 38, and everything below that would be the major versions affected). This is roughly 4% of our users.