Try adding this code:

#urlbar *,
#FindToolbar *,
#search-container * { font-size:11pt !important; }

#urlbar .textbox-input-box,
#search-container .textbox-input-box, 
#FindToolbar .textbox-input-box
 { font-size:12pt !important; font-family:"DejaVu Sans Mono"; }

To the userChrome.css file which is located in the Chrome folder in your Profile folder (For more information go to Editing configuration).

You can also stretch the address bar by customizing the toolbar; re-sizing the Search Bar and removing unneeded buttons.

Add this code:

#urlbar *,
#FindToolbar *,
#search-container * { font-size:11pt !important; }

#urlbar .textbox-input-box,
#search-container .textbox-input-box, 
#FindToolbar .textbox-input-box
 { font-size:12pt !important; font-family:"DejaVu Sans Mono"; }

to the userChrome.css file which is located in the chrome folder in your Profile folder (for more information on how to get there go to Editing configuration).

If there is no chrome folder in your Profile folder, create a folder named chrome and create a plain text file inside the folder named userChrome.css.

You can also stretch the address bar by customizing the toolbar: re-sizing the Search Bar, thus making the address bar longer, and removing unneeded buttons or bookmarks from the toolbar to make more space for the address bar.