This problem with the font sizes sounds like an issue with the page zoom. Firefox remembers the zoom levels you set on pages or sites, so that would explain might why Facebook looks smaller on the home page. If you have Firefox version 29 or higher, then when you click on the Firefox menu on the right side of the window (the 3 horizontal bars icon), you should see a percentage in the center column near the top of the pop-out menu. Clicking the minus sign to the left zooms out, clicking the plus sign to the right zooms in. Clicking the percentage number in the middle resets the zoom level to 100%. You can also change the zoom in the View menu, near the top-left of the Firefox window. (If you don't see the File, Edit, and View menus by default, just hit the Alt key and they should appear.) From there it's View > Zoom > Reset. You can also zoom in and out with the keyboard shortcuts: "Ctrl +" to zoom in, "Ctrl -" to zoom out, and "Ctrl 0" to reset the zoom to default.