1. Open Firefox
2. Open Activity Monitor
3. Go to http://wordpress.com, let it fully load.
4. Watch Firefox CPU. If you do nothing the usage is around 1-3%
5. Go to http://google.com. I see 1-3% usage.
6. Go to http://news.bbc.co.uk/ I see usage of 1-35%
Safari is 5-40%. I assume it’s that single line of latest news.