If you want to install the Google Toolbar then just go to Google Toolbar for Firefox page and from there click on the download link. When you do that then you will get "Terms and conditions agreement" page. Read that properly before proceeding and if you agree with that then only click on "Agree and Install" button.
When you do that then firefox will show you warning as given below in the ltop of the firefox page. This will be like “To protect your computer, Firefox prevented this site (toolbar.google.com) from installing software on your computer”. You will have to click on the Edit Options button that is there on the right of this warning and this will open "Allowed Sites - Software Installation" dialog.
Ensure that the address is same as toolbar.google.com and then only click on Allow button. It is very important to be careful while adding sites on this list. This is because there are some of the sites which will make you to install software that you do not require on your computer.
Once you are completed with adding toolbar.google.com then close the "Allowed Sites" dialog. After that you will have to click on "Agree and Install" button from the "Terms and conditions" page. It will show you the "Software Installation" dialog. Here you will have to verify again that what you are going to install google-toolbar.xpi from http://toolbar.google.com/
. Click on the button Install now.
Now you will see the "Extensions" dialog and the download of about 358 KB on the Google Toolbar extension. When you restart the Firefox then it will start with the actual installation of the extension. So just close all the Firefox window and start it again. When you do that then this will show you the post-install dialog and from there you will be able to choose search site and also make the Google as default search engine for the firefox. Other than this, it will also add toolbar.google.com to the list of allowed software providers. Now you are done with the installation. Once you are done with the installation then you will be able to customize the toolbar in the way you want.