Although badly-written extensions do cause a lot of problems, please don't assume they cause every problem. It's not always true and comments like these give the impression that Firefox developers simply pass the buck every time somebody complains.
Having said that, if someone is experiencing bad performance and they do have extensions enabled, temporarily disabling them is a good diagnostic step. If performance improves, it's clear that it's an extension at fault, and then bisection can be used to work out which one. Otherwise, it's clearly a Firefox problem and a bug report (bugzilla.mozilla.org) would be very helpful!
