Windows only: The free Orca Browser is a web browser based on the Firefox 3 core with the same speed and features of Firefox in addition to a handful of extra features. Those extra features include a built-in ad blocker, flash blocker, auto form-filler, and perhaps most useful, an online storage account that syncs your bookmarks, auto-form information, and more with an online account. Orca is made by the same folks who developed the freeware Avant Browser, which is based on Internet Explorer with an emphasis on speed and improved features.
Orca has a consolidated chrome to optimise screen real estate and comes boxed with several different themes. The biggest drawback I've noticed off the bat is that it doesn't seem to support Firefox extensions. (It appeared to be working when I tried installing some extensions, but still no luck after restarting.) If you're attracted to Firefox because of its extensibility, this is sort of a deal-breaker. On the other hand, if you use Firefox for the security and speed, don't want to install extensions yourself, but want more functionality, Orca may be worth a look. The Orca Browser is a freeware, Windows only. It's currently in alpha, but a more official release is slated for August.