Thanks - that cleared things up!
Turns out that what made it initially crater was having Cookie5 try to manage Firefox. Chrome and Safari: no problem. Selecting Firefox brought the installer down (repeated attempts) with the Apple crash reporter screen being displayed. Selecting Firefox after Cookie5 was installed still crashed things, but without any further popup.
Cookie5 Firefox management worked on the original / purchasing machine, and upgrading to the latest version of Firefox didn't seem to help. Thankfully, we almost never use Firefox (on either machine), and from memory Firefox's cookie management was always pretty good. As such, this issue is now somewhat of a moot point for us. (If however you would like me to send you a crash dump, I can recreate that fairly easily.)