Taking a queue from Nick Thomas (Post), and as he so deftly notes. This breakage is also for SeaMonkey. To quote him (while replacing with the SeaMonkey Info [in bold])
There was some accidental breakage in the Mac builds of Nightly last Friday (Sept 16th), which results in a crash when you try to update your build. The revision & buildID for the broken build is
- ad202468df63, 20110916003001
You can check what you have by loading about:buildconfig, or evaluating navigator.buildID in the Error Console.
The solution is to download the latest Nightly build from http://ftp.mozilla.org/pub/mozilla.org/seamonkey/latest-comm-central-trunk.
Thank you Nick for calling out this issue!