Firefox 3 has an oddly sized back button. It's big. Some people may like this since it's a commonly used button and the increased size makes it's easier to hit. But if you don't use the back button a lot. Or you have a sense of design that is easily offended, you may want to take step to try and correct the odd look that results from having this single large button.

From the Firefox 3 menu bar select View > Toolbars > Customize. When this opens the toolbar customization pane, check the box next to Use Small Icons and then press the Done button to save your changes.