SeaMonkey was released first time in 2006 and is an open source browser where people are contributing in different ways to maintain and enhance the product. Is made based on source code used by Mozilla Firefox, but there are many differences between these 2 browsers, especially regarding design and usability, Mozilla Firefox being better.
The program supports multiple tabs and these can be arranged with drag and drop. New items (sites) can be added to Bookmarks bar with drag&drop too.
The omnibox is working slow, but at least you can manage default search engine.
It supports to customize default font, check spelling, includes a download manager and can be synchronized across devices.
The design looks classic and old, the browser doesn't provide a default search box. But SeaMonkey supports all extensions and themes available for Firefox.
The good thing is that SeaMoney it support Firefox extensions, but the program lacks regarding design and usability.