Funny you should say that. I regularly use Firefox and Chrome on two very different machines: one, is an anemic laptop with a Pentium T4200, the other a desktop with 8 cores and lots of memory. On the weaker machine, Chrome is visibly snappier - and never slows down at the end of the day. Firefox seems quite spry in the beginning, but quickly becomes visibly slower - not its rendering, but its general reaction, the awful, XUL-based interface. By the end of the day, right-clicking on a page and/or opening a new tab has a very visible, very annoying lag and the overall reactivity has decreased greatly.
It's not a problem induced by some esoteric extensions (I only use Ghostery), I have enough memory (8GB, and the system memory load never goes beyond 50-52%), I only read text (lots of pages, though), no video, no games, flash is disabled via click-to-run.
The faster/newer machine exhibits the same behaviour, it only takes a while longer for the lag to be apparent (due, obviously, to the increased computing power).
I've always been a supporter of Firefox, I've been using it continuously since the 0.x era (in its Phoenix incarnation), but I'm not blind and statements like "FF is faster than Chrome" simply make no sense to me (and my browsing habits).