If you find that after upgrading to Firefox 3, your toolbars, menu text, and page content are much larger than they should be, follow the instructions in this article.
If your Firefox window's text toolbars, menu text, and page content are normal size, but the window is too large, see the Changes to toolbars and window sizes are not saved article.
To restore Firefox to a normal size:
In the address bar, type about:config and press EnterReturn.
If the layout.css.dpi preference is not shown
Firefox will be sized normally after you restart it.