First, and foremost, probably the biggest selling ( so to speak, it is FREE after all. We like free stuff, yes?
This baby also shows more page per window. Up to 4% more than IE in fact! And hit F11 to knock out a couple of the toolbars, and the status bar on the bottom for even MORE viewing space.
You can add features to Firebird to extend its extreme usefulness. So far have personally added a built-in calculator, client, extension that saves files to different folders automatically depending on extension, a toolbar that assists in web development ( think built in code validator, element outliner, table outliner, etc. ), and this little thing called gnusto that allows you to play z-code adventure games, which admittedly I have not tried yet.
There are also built in accessibility features, most up-to-date standards support of any browser, default configuration to get rid of annoying browser tendencies, etc. Even though the download size is 6.0 MB, it is well worth it.
If that doesn't convince ya, go here: Why switch to Firebird?