With the release of Firefox 3.5, comes some new features and some annoying ones.
One annoying feature is wasting un-necessary real estate to include a new tab button on the tab bar.

All the button does is add a new tab when clicked, which is really not needed since you already have three options to add another tab, with the easiest method by pressing CTRL+T on the keyboard (the other two ways are double clicking on an empty tab bar area or right clicking on the tab bar and selecting New Tab).
A fourth method is not needed (yet another reason why Firefox is bloated).
Fortunately Firefox is customizable and removing the tab button requires a simple fix.
read more: How To Remove The New Tab (Plus) Button In Firefox 3.5