All websites that heavily rely on Javascript just work better on Chrome for me. Google products, OSM, streaming sites, chat applications, they just all work quicker and smoother on Chrome. Even my own code seems to run into more performance caps on Firefox depending on the behaviour it exhibits.
On plain websites, there's no real difference. When it comes to (messy) JS, problems pop up more in Firefox than in Chrome.
I don't blame the Spidermonkey devs for this, of course, because people just need to write better (less) JS. However, in practice, you do see the difference when you try out Firefox.
Out of curiosity, what makes you think that the culprit is the JS engine?