While a great start, it's important to remember that Firebird is still unfinished and is in the beta testing phase.
Firebird currently lacks an installer. It just runs from the folder you extracted it to (i.e. if you saved it to the desktop, and unzipped it there that's where the program files are located). You can move the Mozilla Firebird folder to your Program Files folder, or any location you choose. Create a shortcut by right clicking the Firebird icon, and selecting "Send to Desktop (create shortcut)".
My favorite feature is the tabbed browsing. Here's how I use it: I have a folder in my bookmarks (favorites) I call daily links. These are about a dozen sites I visit every day. When I click on Bookmarks, and then Browse this Folder, the last choice is "Open in Tabs". Each site is in this folder is opened in it's own tab accrosss the top of the browser window. Because the sites are loading in the background, they're ready to view when you click its tab. Even though I'm on broadband this is a nice time saver. I think it'd be huge for dial up.
One feature I just recently learned about is Type to Search. Just start typing and the matching link will be hilited in your browser window (you don't need to use your mouse to click a link).
If you've used IE for a while, you'll probably miss your stored passwords. These can be saved in Firebird too, but aren't imported automatically. You'll also likely need to download Flash, and Shockwave plug-ins when visiting sites that use them.
You'll probably also notice a few pages may not display correctly. This is usually due to the differences in the way IE and Firebird handle coding errors, or possibly a page coded only for IE. Also, beta software isn't going to perform perfectly.