Okay... so the question is, where do I go for help?
Firebug refuses to help me because this is a custom build of Firefox that doesn't have a Crash Reporter and so they can't find where the issue is.
Firefox won't help me because they don't support custom builds of Firefox (all they told me was that they should figure out a way to get Crash Reporter working for custom builds).
And now the package maintainer won't help me because I'm using an "unstable" version of the extension.
As I said in my report, this is working fine in the 32-bit Mozilla release, and thus it is highly unlikely that the Firebug team will ever try to diagnose and resolve this issue before 1.5 releases. The 1.5 alpha builds are extremely stable and have some features that really make my life easier.