Perhaps a stupid question, but would it be safe to assume my issue is my own fault? Like I mentioned earlier, the troubled machine has had the ubuntuzilla.py installer at one point but the fresher install has never had it.
When I did the fresh install, I simply Googled Ubuntuzilla and found the new apt repo, and when I was finished there I went to the other machine and just removed the .py and added the repo.
Until tonight I'd never seen the plainly wrote out instructions -- "Current users of ubuntuzilla should run "ubuntuzilla.py -a remove -p yourpackage" before installing these packages, to undo the local diversion of /usr/bin/(package) links."
So in theory (had I not been an idiot and just decided to reformat and install a fresh 10.10 before reading a little deeper) could I have just reinstalled ubuntuzilla.py, then correctly remove it to fix the issue I was having, since it looks like there were some some dpk-divert errors in my log?
yes, it your case, it seems that all you'd have had to do was to remove the local dpkg diversion for /usr/bin/firefox, and then you'd have been ok....