One of the most noticeable change in Firefox 3.5 is the tab bar. It's always there, even if you only have a single tab open. If you can't remember the shortcut to create a new tab (Ctrl + T), I guess having the bar always there may be of some help. But if you don't need it there, it's otherwise just taking up space when you are looking at a single tab.

There is a setting that allows you to change this behavior so that when you have a single tab open, the tab bar is not shown. To set this go to the Firefox Preferences and select the Tabs tab. Once there you'll see an entry called Always show tab bar. If you uncheck the box next to this, Firefox 3.5 will no longer show the tab bar when you are looking at a single tab.