It uses the Mozilla Engine for rendering the webpages, they are almost identical.
You can even use most of the mozilla plugins in Firebird too.
The Preferences Dialog is a bit more limited than the Mozilla one. If you want to have full control, you can install a extension that allows you to set every detail you want.
The extension i love most is the
web developer extension.
As is said, Firebird doesn't have a sidebar and you'll miss the newsgroup, mail and calendar from the mozilla suite if you're used to them.