I typically use Chrome, but recently tried giving Firefox another chance after not having used it in a year or two.
It has some nice features that make it very attractive over Chrome (like better management for many tabs open at once), but I have found performance to be absolutely terrible much of the time.
I go overboard with tabs sometimes so I expect it to suck memory, but I find that even with only a few tabs open it is frequently using 10% or even up to 30% CPU on the regular Firefox process or usually the Plugin Container process.
When this happens Firefox becomes very sluggish, for example it may take a second or so for words I type to appear in a text box. This typically happens after it has been open for a while. Killing the Plugin Container process and reloading the tabs or just restarting usually makes it better but only for a while.
This is across three different computers and still seems to happen with all non-essential add-ons and plugins uninstalled. One computer is a cheap laptop but the other two are high-performance machines with fast processors and Firefox still becomes unusable. It happens with updated versions of both regular and developer editions.
There also typically aren't any obvious demanding scripts running on any of the pages.