

I still had no idea it was 64bit when I installed it.
MOZILLA THUNDERBIRD FOR WINDOWS 7 PRO PC
Since Mozzilla never advertised the 64bit when I reformatted my PC and downloaded the latest Thunderbird, it was Version 68. Folder switching seems a lot faster and folders display emails more quickly to before. I don't have scientific proof but it seems to me that the 64-bit version of Thunderbird is more responsive than the 32-bit one on my system with a good number of emails in the database (120k). Go back to the 32-bit version if you notice any issues afterward. Thunderbird should start up normally and you can check that you are running the 64-bit version now by selecting Help > About Thunderbird. Note though that both installations will use the same user data.
MOZILLA THUNDERBIRD FOR WINDOWS 7 PRO INSTALL
You could select custom installation at one point to install it to a different directory than the 32-bit version so that you could go back to the 32-bit version without installing it anew. The installer does not reveal to you that you are installing the 64-bit version of Thunderbird.

You can also use MailStore Home to backup all mails locally.

Since you are installing a version that is not supported officially, you may want to back up the Thunderbird data folder or even the entire system partition before you start the process.
