That's part of the problem. I'm a huge fan of Firefox, but I shouldn't have to manage these things in order to keep my browser fluid after six months of regular use.
However, there are still extensions that cause issues, and the odd bug that is an actual issue with how we do things.
The reason he recommends creating a new profile and syncing stuff over is because it's sometimes easier than to figure out what's broken on a particular system that has a pre-Firefox 4 profile.
Here's a step-by-step instruction on how to do it — if your Firefox is slow, this will most likely fix it: http://limi.net/articles/firefox-preferences/