Tab Hierarchy: In Gmail or Google Reader, click an external link and a new tab is opened, as in Firefox 1.5. What’s new is that when you close that second tab, focus returns to the tab that opened it (e.g. Gmail or Reader). That’s almost always what I want so it saves me mousing.
Close Tab Via Direct Manipulation: Firefox 1.5 had a single tab close control – way over on the right hand side of the window. If you wanted to close a tab via mouse, you had to click the tab you wanted to close, then mouse over to the right to the close control. Now if you want to close a tab via mouse just click the close control on that tab. This is a Safari feature I’ve been pining for.
Minimum Tab Width: If you opened lots o’ tabs in Firefox 1.5, eventually they’d get so narrow that the favicon would become obscured. In Firefox 2.0 tabs reach a minimum width then don’t shrink any further. There are overflow controls naturally, so you can still get to all your tabs. This new behavior represents a nice trade-off.
OK so that was three reasons.