Compare K-Meleon on Windows to Chrome and Firefox to Chrome. Switching between tabs, opening menu, etc is slower on Firefox while the difference is less noticeable between K-Meleon and Chrome.

And on Linux, Chrome's custom toolkit feels much faster than the Firefox XUL. Everything from opening new tab, switching tabs, menus, etc. And using Electrolysis doesn't help. So it's an issue with the XUL toolkit.