I decided to give it a try. It's certainly an improvement over Mozilla 1.3 (the last version I used under Windows).
There's a couple things that I've noticed:
Like Mozilla, Firebird uses none of the standard Windows GUI libraries, which means its interface is a little different than a standard Windows App. (Though it's much improved over Mozilla.) I'm sure this is done for portability between OSes. Unlike Mozilla, Firebird's GUI is fast as well.
Very clean install (in fact no install, just unzip and run the .exe). It looks like Firebird only uses its own libraries. (This makes it fast, but does add to the download size, though they have managed to keep it very small.)
Compared to IE, Firebird is definately faster. It seems to render most pages pretty well.
Compared to Opera, it's almost as fast. Opera is the only browser I know of which will display individual elements in an HTML table before it has downloaded the whole table. For modem users this means Opera will start displaying content on some webpages before Firebird or IE. Firebird finally has an integrated Google search, Cookie Management, tabbed browsing, and integrated search functions. Those were some of the reasons why I use Opera. I think Firebird renders pages better than Opera overall. (Though it does seem to have a bug with the cursor on this text entry box...) Opera still has some features that Firebird lacks: The ability to identify as IE (which allows Opera to work with some pages which supposedly require IE), referrer logging control, the ability to open only requested pop up windows, mouse gestures, the ability to disable animated images (flashing banner ads), a quick preferences (F12) window, and a text only browser mode to name a few.
Overall, I'm pretty impressed. It definately has potential and should be very good by version 1.0.