How to customize icons in firefox
Add more icons to your firefox Toolbar
To add more icons to your firefox toolbar such as printing, downloads,history or full screen, go to View then click Toolbars then click Customize and you can drag and drop icons to the top of your browser toolbar.
To give you a more screen space in firefox, go to the View, select Toolbars,then click Customize option and then check the Use Small Icons box.
And also configure the show option to change the look of your firefox.