This morning my PC's speed began to slow down (I have VirtualBox, Firefox, jEdit, CoreFTP and winamp on load) the usual noticeable lagging of the mouse cursor's movement irritated me, obviously one of the applications was eating a big chunk of the process, so I hit CTRL + ALT + DELETE, to check what was going on...
The CPU usage shows 100%, then I checked which processes has the highest memory usage, the culprit was firefox - BUT I wasn't really browsing memory heavy webpages, still I checked which tab should I close, then I noticed Yahoo! Mail has an animated flash ad (about 400x400). I closed that tab, then checked my CPU usage again, eventually it went down to 13%.
This remarkable discovery Ü lead me to download a firefox plugin to block ads.
Lo and behold! The Adblock Plus by Wladimir Palant
It's very easy to install and supports a lot of filter subscriptions.