There are ways to increase the height of the URL bar by dropping code userChrome.css file. This line should be added to Firefox userChrome.css file. The URL bar expands to take the bigger text:-
/*Change address bar font, color and size*/
font-family: Tahoma Bold !important;
color: Black !important;
font-size: 12pt !important; }
Alternatively (and more easily), just install the following extension "Make Address Bar Font Size Bigger 1.0.0":-
Again, making the text bigger increases the URL bar height.