
Second, that 64-bit versions of Thunderbird may perform worse than 32-bit versions under certain circumstances. While that does not mean that 64-bit applications won't run just as good as 32-bit versions of Thunderbird, you may run into issues that are exclusive to the 64-bit version. First, that 64-bit versions are not officially supported at this point in time. You need to be aware of a couple of important things before you migrate. The page lists the version of the client and whether it is 32-bit or 64-bit.

Mac users get the 64-bit version of Thunderbird automatically, and Windows users get the 32-bit version of Thunderbird. When you open the "all" download page on the Thunderbird website, you will notice that only Linux users get to choose between 32-bit and 64-bit applications.

While it is possible to grab the 64-bit version, users have to actively search for it to download it as there is virtually no reference on the official Thunderbird website that indicates that such a version exists.
