Firefox 5 introduces a new feature that changes the way tab bar behaves when you close a tab. In Google Chrome, when you close a tab, the remaining tabs do not immediately resize. This allows you to keep closing tabs without having to move your mouse. Once you mouse off the tab bar then the remaining tabs will resize to adjust to the space on the tab bar. This feature has already been implemented on Firefox 5 Aurora as well as Beta. For those who prefer the old behavior, I am certain there is an about:config tweak, however I am unable to locate it at this time.
Firefox 5 is scheduled for release on June 21, 2011.