I'm running Firefox 7.01 on a 4 year old machine with Ubuntu 11.04. I routinely have 30+ tabs open an never had a problem with speed. Maybe your friend's computer has other woes
These "I have X tabs open" comparisons are very unhelpful. Different sites take hugely different amounts of memory in just the same way that different desktop applications do.
If you want to make a even-slightly quantitative comparison you need to at least start by loading the same set of sites. Then you need to worry about them sending different content (e.g. different adverts) to different browsers.
In the end most people go by metrics like "how responsive does the UI feel", which is fine, although since it's a perception thing it can be influenced by factors other than the actual speed e.g. preconceived notions set by marketing.
