We are planning to discontinue support for 32 bit Macs for Tor Browser. We're doing this for two main reasons. First, Apple itself no longer supports 32 bit Macs. The only remaining 32 bit Mac users are on OSX 10.6, which Apple ended support for in February of this year. Second, 64 bit software has improved security properties by way of improved address space layout randomization (ASLR), which makes exploitation more difficult.
This transition will happen in phases: First, the upcoming 4.5-alpha series will only be available as 64 bit builds. We will continue releasing 32 bit versions of the 4.0 series until the 4.5 series is declared stable. When the 4.5 release stabilizes, all support for 32 bit Macs will end. The 4.5 release series will be stable "when it's ready", but we know we need to perform at least two more releases in this series to properly exercise the new security features in the updater. This means that 32 bit Mac users likely have a month or two to decide what to do.