Like always when people start comparing system performance, there are so many variables at play (CPU make / model / age / cache / n cores, bus speed, RAM type, OS / distro / package versions / compiled flags, swap space, running daemons / services or other background processes, number of browser tabs and even the specific sites left open, etc etc)....
But for me it's the Flash plugin container for Firefox that really does the damage when it comes to concurrency. A few Firefox tabs open running YouTube (or another video streaming service) and my fan already starts spinning up louder than a revving motorbike. If I have much more open aside Firefox then the system will just lock up without warning. I suspect Flash is thrashing both CPU and RAM though but it's consistent enough behaviour that I tend to avoid it rather than investing time debugging and fixing (though since it is Flash that kills it, I suspect any such "fix" would just be altering the behaviour of the softwares end user (ie me) anyway).