Hi Tomas. I don't have a PC to test on but I did a little testing on my Macbook.
Using the mac Activity Monitor, while having a couple tabs open in Firefox, my cpu % is at 5% and memory usage at 400MB.
I opened the Aesop demo at http://playground.aesopstories.com/ and clicked through all the stories one right after the other. My cpu % went up to about 75% and memory usage to 540MB. Immediately after closing the tab, usage levels went back to normal.
Next I went to http://www.cnn.com/ and did the same thing. On CNN's site, clicking through articles, my cpu % went to 110% and memory usage to 580MB. Immediately upon closing the window, usage levels went down to normal.
It is my opinion that Firefox in general is notoriously bad for being a memory and cpu "hog". Even my own WordPress site slows down in the admin. It's a good idea to close Firefox every once in a while to clear the usage and start fresh.
I will pass your note along to our team so they are aware of your concern. Thanks for letting us know!