If you want to try it, you'll probably want to run Firefox 3 and Firefox 2 at the same time. Only this time you should remember to rename Firefox 3 so that it doesn't override your install of Firefox 2.
I'd blogged about some of the changes previously if you care for some screenshots of some of the new features (Mozilla Links covers a good number of new Firefox 3 changes too):
- Resumable downloading: users can now resume downloads after restarting the browser or resetting your network connection.
- supports Growl for notifications of completed downloads and available updates.
- Easier password management: an information bar replaces the old password dialog so you can now save passwords after a successful login.
- Optimized Open in Tabs behavior: opening a folder of bookmarks in tabs now appends the new tabs rather than overwriting.