rescuetime.com slows down firefox

I was wondering FF3 using ca. 40-50% of CPU (that means a whole core!) the whole time in the last days. I searched for the origin of this phenomenon and now I found it:

As long as I have rescuetime.com in one of my tabs (doesn't matter if this tab is active or even if I use FF3 at the moment or having it run in the background) the CPU usage is that high! As soon as I close the tab the usage by FF3 rapidly goes down to 5-10% when using it and ca. 2% when using another program and having FF3 running in the background.

So how can it be, that only having rescuetime in a tab causes such a high CPU usage? I mean I don't even use it. I can have google opened in 50 tabs and it does not use any CPU usage. Why does rescuetime.com?
 
sad I’m frustrated
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