In fact as far as I can tell each new version of Firefox seems to leave a smaller footprint than the previous.
When Netscape packed up I continued using it for as long as possible, perhaps out of some feeling of loyalty to the browser. But as time passed I realised that I had to move on as well. Loyalty doesn't really work with software especially when newer versions are released, because the new versions offer enhancements, security improvements, and so on.
In my view, and especially with browsers, it is always better to move up with each new version.
I have heard some say that Firefox 2 is better than Firefox 3 in terms of speed because Firefox 3 got bloated. Is there an truth to this?