You are pointing to individual close buttons on two tabs. The "Tab button" is the one that looks like an ink bottle (supposed to look like a new tab simulating top of a manilla folder).

With "Customize" you can rearrange to buttons on the toolbars (see item # 4 in the following...).

You can make Firefox 4.0.1 and Firefox 5.0 look like Firefox 3.6.17, see numbered items 1-10 in the following topic Fix Firefox 4.0 toolbar user interface, problems (Make Firefox 4.0 look like 3.6)

Also see item #35 if you liked the way tabs were closed with a single button in Firefox 2 or just want a better way, especially if you have 40 tabs open.

You can find the New Tab button showing as a '+' on the Tab bar.
You can open the Customize window and drag the New Tab button that shows as a plus sign (+) from 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.
If you can't find the new tab button then click the "Restore Default Set" button in the Customize window.
If you want the New Tab button at the far right end of the Tab bar then place a flexible to the left of it.

• New tab toolbar button: https://addons.mozilla.org/firefox/addon/new-tab-toolbar-button/