A nice little post about usability testing of tabs in Firefox:
Here at Google, we did some usability studies on the tabbed browsing feature. Our usability analyst designed a study to see how well people responded to tabs and how easily they were able to switch between them and close them.
One of the things they are experimenting with is putting close buttons on each tab. Safari already does this and I have to agree, it's a much better way of handling tabs, rather than having the one close button on the right, distanced from the tabs themselves.
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