The "average user" you're talking about, first of all, probably doesn't even use Firefox. Most "average users" are still using Internet Explorer, believe it or not.
Agreed, but if they are using IE I disagree that they'd be using Linux at all.
With that in mind Linux users are generally going to be a bit more techy minded and will want to be up-to-date so they are not vulnerable to any known security issues and so things generally improve (speed, features etc).
By next April (Dapper) Firefox 2.0 will be very close if not already out - so for those who like to only use apt-get that will basically mean they will never get to use 1.5. Or, if 2.0 isn't out by April it will most likely be released shortly after the Dapper release, which means the same problem is going to arise again - people will be stuck with an old outdated and possibly dangerous version of Firefox.
My point being some kind of solution needs to be put into action instead of the attitude of users can wait, users are dumb, they probably don't even know what version of Firefox they are using anyway. That may be true for Windows users but in general I'd say Linux users are more aware of what they are using and what version it is.
So basically why can't Firefox 1.5 be put into the backports like the first user suggested with no links to the current firefox (e.g. it doesn't overwrite 1.07). That is basically the same as the how-to's suggest and provides a way for users who only install something if it is in apt-get.
After all the overall aim of Ubuntu is supposed to be ease of use. A distro for everyone that everyone can use - however I don't see why easy to use should also mean outdated and potentially vulnerable software.