A while ago, I replaced the official Mozilla build of Thunderbird I had on my Mac with an optimized-for-Intel build I found here.

Taking the process one step further, I compiled it myself from source. This allowed me to change one thing that annoyed me in the application. Thunderbird puts subject lines between brackets when you forward a message. It is not a configurable option. This is hard-coded into the application in the file nsMsgCompose.cpp (lines 2017 and 2018) and in mimedrft.cpp (line 1350).

One other annoyance I had with Thunderbird on MacOSX is that it displayed the date in MM/DD/YY format, rather than the DD/MM/YY format we use in Europe. This happened regardless of the date settings on the Mac, and irrespective of the official build I used.  Bingo this time. Recompiling Thunderbird solved this issue. My guess is that the application takes the defaults of the platform being used for compiling, rather than the run-time defaults.

Finally, my Thunderbird.app is now 31Mb, rather than 52Mb for the official build. As a result, it starts and runs visibly faster.