I remember one time wwhen I was browsing reddit for a few minutes (booted righ before, doing nothing else, fedora os, minimum stuff installed, no unusual activity in the process list, automatic updates disabled) and noticed my fans spinning a lot, so I opened the system monitor to check what was happening. It was showing high cpu usage in a firefox process, and it registered 40bg of downloaded data!

It never happened again, so I guess it was some sort of bug they fixed, but still, it was something so bizarre to see.