Firefox: The default setting in Firefox 3.5 is to always display tabs, even when you're only browsing a single page. Save screen space by saving the tabs for when they do some good.
Over at the Firefox-centric blog Firefox Facts, they share a simple fix for the "Why is there suddenly a @*$% single tab in my Firefox?" complaint:
From within Firefox, go to Tools, and then Options. Now, got to the Tabs menu, and un-check the "Always show the tab bar" selection, and hit the OK button.
From there out, you'll only see tabs when you're browsing multiple pages and not when you only have a single page open. Have some Firefox-related tips to share? Let's hear about them in the comments.
Hide the Tab Bar When Only One Page is Open [Firefox Facts]