As a novice, I enjoyed 4.0's new tab definition option.
Now I've installed 5.0 & can't find a way to define new tabs.
Alla svar (1)
You can find the New Tab button showing as a '+' on the Tab bar.
Open the Customize window via "View > Toolbars > Customize" or via "Firefox > Options > Toolbar Layout"
If you can't find the new tab button then click the "Restore Default Set" button in the Customize window.
You can open the Customize window and drag the New Tab button that shows as a plus sign (+) on the Tab bar on another toolbar and it will become a regular toolbar button like the New Tab button that you have in Firefox 3 versions.