That's rather interesting as I can't reproduce your problem even if I tell Firebug to change the page font to something similar to as yours appears, and reduce the window size. Even zooming in doesn't have that effect though I assume that's not the problem as you experience it in both IE and Firefox.

Try pressing Ctrl + 0 to reset your zoom level (in Firefox). Any idea why your browser would be using Arial as the font instead of Tahoma or even its defined primary fallback, Verdana?
Perhaps check this options window in Firefox (Tools -> Options -> Content tab -> "Advanced..." under Fonts & Colours) and ensure that the checkbox for allowing websites to choose their own fonts is ticked.
