If you mean you got thunderbird from Thunderbirdownload - Mozilla x86-64
then they are definitely 64-bit. If you open task manager and look at the thunderbird entry you wont see a *32 moniker beside it.
Because the versions of thunderbird/firefox on that site are alpha/beta builds they dont use the proper names of the programs but rather the project names. Shredder is the name of a particular project of thunderbird, just like shiretoko was for firefox.
So just to confirm, these are definitely 64-bit and you can check for yourself as described above.
One thing to bare in mind is that 32-bit plugins wont work for the 64-bit version of the programs.