There is a very noticeable difference between Firefox 3.0.11 and 3.5. 3.5 Uses a new javascript engine that is much faster.
Double the speed from 0.2 seconds to 0.1 seconds for javascript, when javacript performance is comprises a minority of site loading time isn't very noticeable.
Firefox 3.5 is indeed a lot faster than 3.0. I benchmarked both myself and noticed a respectable increase
without visiting benchmarking sites.