Firefox 3 for Linux integrates with the current GTK theme. If you don’t want to run beta versions until the final release, you can get some of the same integration in Firefox 2 using a theme.
The “GNOME” Firefox theme is available at the Firefox Add-ons site. Once it’s applied, Firefox will use GNOME icons and native-looking GTK tabs. The tabs even change colour to match the current theme’s colour.
Regrettably, the theme replaces the gradient on tool bars with a flat colour. So if you want perfect integration you will have to use the Firefox beta or a true GTK browser like Epiphany.