Via Slashdot came the news that version 1.8 of the Gecko engine used to render HTML in Firefox 2, Thunderbird 2, etc. was being end of lifed. Now I have still a few computers which I'm responsible for that have FF2 on them, just because that's what the users were accustomed to. So I searched around and found this: Firefox 2.0 Classic Theme for Firefox 3.0. So I will be installing FF3 with a FF2 skin there.
Also, the "news" was misleading (what a surprise - FUD on Slashdot :-)). Thunderbird is not going away, nor do you have to update to an alpha version of it. They will be supporting Gecko 1.8 with security patches for some time, it's just that new features won't be added (which isn't so critical in the case of mail clients - HTML mails are evil anyways :-)). The new Thunderbird will be released sometimes next year with the new Gecko engine, but there is no need to rush the upgrade (or at least nothing related to this announcement - maybe there are features in 3 which are vital for you).
In conclusion: the sky isn't falling (yet) and always look at the bright side of life :-).
Update: It seems that the theme is marked as "experimental", and thus you need an account on addons.mozilla.com to be able to download it. I found the following account to be working from bugmenot: firstname.lastname@example.org / bugmenot.