Whilst the What's New section of the Mozilla Firefox 0.8 release notes didn't contain anything that leapt out at me, after a few days of use I can attest to the big difference that the small changes have made.
For Windows there is now a proper installer. Although a little quirky (try installing into a non-existent folder) it does make the installation easier than before. The Add Bookmarks dialog has been blessed with the addition of a collapsible tree rather than just one long fully expanded list. The URL suggestions begin with the base URL of a site, rather than just any page on that site, which makes it all eminently more usable. And finally, the download manager now encompasses all dowloads in one window, and includes an overall percentage downloaded.
Plus, with all the bugfixes that have gone in since Mozilla Firebird 0.7, there's bound to be a few sites that you find working just that bit better. So, go get it. Now.