I'm pretty sure I've narrowed this down to a bug with the PA version of TBird. It's crashing whenever I try to load in a new extension, in this case the Lightning and Google Provider extensions. I haven't been able to replicate it in the desktop version of TBird 24 that I downloaded directly from their website. That seems to work fine in Win7.
What happens is I go to Add-ons-->Install Add-on from File, then navigate to the .xpi file, the entire app freezes up when I hit 'add'.
Addendum: I've tried this on two different Win7 machines with the same results.