Right, it turned out my next step was not a chat with Joe Halliwell but instead with my good friend and mentor, Dolan O'Toole, and what I thought might be a voyage of discovery into how to make my installation of firefox run like a normal program turned out to be just a short trip of discovery to finding out that I hadn't actually been looking in the right places.
After helpfully pointing out that I "sound like a linux newbie :)" (yes, yes I am), Dolan explained where I'd gone wrong: "You followed the instuctions as they are. The only thing you weren't aware of is that most of the time, someone would have done a build of a program for ubuntu and put it in an apt repository, usually on launchpad. I usually search for something like "firefox 5 ubuntu 10.10" if I need to install something before I resort to manual installations."
Good tip, and here it is:
So – I have learned that it's not quite as straightforward as finding the instructions and then following them – you also have to know stuff. That's OK though. And at least I have firefox the way I want it now. I'm sure there'll be another opportunity to get my hands dirty...