Firefox (versions 3.5 and up, at least) has the ability to zoom in and out of a web page, adjusting both text and graphics, without affecting the rest of the screen. You can access this feature via the menus (View » Zoom In or Zoom Out), and via the noted keyboard shortcuts (Command-equals to zoom in; Command-minus to zoom out; Command-0 to reset).

What you may not know is that you can also zoom via your mouse's scroll wheel. Press Control-Command (Control-Option also works) and move the scroll wheel up or down. The browser window will zoom in or out while maintaining, for the most part, the clarity of the text and images. The rest of the screen does not change.