This is in two parts.
To get some extra standard buttons on your FF tool you can use this add-on for Toolbar Buttons
To drag them onto your toolbar just right click the tool bar and select customize, and you should have the new one listed in the panel. They include standard ones for Google, Wiki etc. If you want to go onto a second toolbar line use it to create a new toolbar, then add a button to it, and make sure the new toolbar is shown from the VIEW menu.
If you want to create your own buttons and add those, you will need the add-on Custom Buttons2
To add a new button select VIEW/ADD NEW BUTTON from the main menu. That gives you this...
The button name is just to identify it. The image address is to upload an image for the button. This one is for a button to take me straight to latest threads on CiC, so I use my avatar image. The code is to action the button.
To start a webpage in a new tab use this...
To start a webpage in the same tab use this
getBrowser (). addTab ("http://www.cambridgeincolour.com/forums/latest.php");