Image via CrunchBase

SeaMonkey 2.0 contains the following major changes relative to SeaMonkey 1.1.18:

• SeaMonkey uses the same internal platform as Firefox 3.5.4 now. Due to this change SeaMonkey is now much closer to Firefox as far as user profiles, add-ons and functionality of user interface elements are concerned.
• The default location of user profiles has changed. Legacy profiles, e.g. from SeaMonkey 1.x, or even Thunderbird profiles, can be migrated automatically, excluding third-party add-ons (extensions and themes). The old profile will not be touched (instead, copies are made).
• The new Add-on Manager allows to install, update, disable and remove extensions (add-ons), themes and plugins using the same mechanisms Firefox uses. This should ease extension management and development considerably.
• Session restore (crash recovery) has been added. Now, when SeaMonkey crashes, all your browser windows and tabs will be restored automatically or on request (including data entered in web forms!). In addition, users can select to always restore the browser windows and tabs from their last open session when starting SeaMonkey.
• Reopening closed browser windows (Undo Close Window) is now possible.
• History is now stored in a better database solution, so that it can easily keep more information, and the location bar determines which addresses to autocomplete and/or show in the drop down menu with a smarter algorithm based on what you type, searching in both URL and title of visited pages by default.
• Toolbars in the browser, main MailNews and message compose windows are now fully customizable, including icon size and whether to display icons and/or text for every major toolbar, all available via their context menus.

http://www.seamonkey-project.org/releases/seamonkey2.0/