Keramidas talks about font substitution in Firefox and other GTK+ base programs.
The DejaVu font family is a very popular font collection for Linux and BSD systems. The font package of DejaVu includes a condensed variant; a variation of the same basic font theme that sports narrower characters.
The difference between the two font variants is very easy to spot when they are displayed side by side. The following image shows a small part of a Firefox window, displaying news articles as part of a Google Reader session: