It's actually fairly complete, but it's a "technology preview", which is code for "stability and performance not guaranteed". So personally, I would also say stick with Mozilla for now as a primary browser, and play with Firebird on the side if you like.
But keep in mind the Mozilla suite is a dead-end. Eventually, the plan is to gradually merge the two together, such that the changes and features present in Mozilla 1.x are present and stable in Firebird. At some point during that process, Firebird 1.0 will exist, whereupon the Mozilla suite will be retired in favor of two standalone apps - Firebird, the browser, and "Thunderbird", the news/mail reader. The current timetable has that happening sometime next year, perhaps mid-2004. And in the meantime, you should see Firebird and the browser side of the Mozilla suite gradually converging. So if you're a Mozilla fan, Firebird is in your future eventually anyway...