I opened up the same 6 tabs in current versions of all the desktop browsers, then took screenshots of the UI of just the tabs themselves. These are from a Mac running OS X Lion.
- You get an average of 26 visible characters per tab if the user has 1-3 tabs open at around 1200px (average for this site).
- All browsers maintain this 26 visible character average until about 5 tabs, then that number shrinks. At 6 tabs you get ~18 characters.
- IE 9 starts shrinking at 4 tabs and at 6 tabs only an average of 3 characters are shown per tab.
- Every browser shows favicons except Safari.
- Every browser puts tabs on the top (above URL bar) except Safari.
- You don't have much room, make it count.
- You can largely count on the favicon to identify your site so don't start with site title, start with the next most significant thing. Unless you have a ton of Safari traffic (remember, your site's stats are the only ones that matter.)
- Safari should get over it and put favicons in the tabs.