Seamonkey , (new repository: )
You can try the Seamonkey Internet Suite, which bundles a Firefox-based browser, Thunderbird-based mail client, IRC client named Chatzilla, and a web page editor named Composer. Seamonkey is an open source version of the hallowed Netscape Communicator internet suite. One big advantage of using Seamonkey is that if the mail client is already open, the browser will open zero delay. The other advantage is that all your browser, mail client messages and accounts, chat settings, add-ons, and passwords are all in one directory. When you change your computer or upgrade the OS, you just need to copy the Seamonkey profile folder. Almost all Firefox extensions work without any problem.
Seamonkey is available for Windows and Mac also. It can be very useful for Linux users who use multiple operating systems. The profile folder works across platforms. 64-bit Linux users should note that the regular download file will not let flash to run properly. The "unofficial" 64-bit build is usually available in the page under the "Other Systems and Languages" link. It works without a problem in all the Ubuntu installations that I have tried. Installation is simple. Extract the archive in the /opt directory and create a soft link to seamonkey file in /usr/bin. You may want to backup any old /usr/bin/seamonkey link that may exist.
This is the modern theme:
Some tweaks to make Seamonkey look and act sleek and fresh:
Right-click on navigation bar, "Customize": drag the google etc search space onto the navigation bar (possible remove the old "search" button by dragging it into the Customize Toolbar window; also may remove or add other stuff visible there)
To make tabs more customizable install Tab Clicking Options
Support for more addons and plugins on mozdev.org. (e.g. context search)
There are few themes available, but the default one is decent, and Kilome has a light fresh look. Also, the new type of firefox themes (formerly called "personas") can be also used to spice up the default theme, and they are very different.