SeaMonkey provides an integrated suite of complete Internet tools, including email, browsing, chat, development, and more. SeaMonkey is also a community developed project.
Expert community: The community that has assembled SeaMonkey knows what they're doing. SeaMonkey is built on top of a Mozilla engine, which has for many years now been the go-to source for many highly popular Internet applications including Firefox and Thunderbird.
All-in-one: The thing that most people will like about SeaMonkey is that it is an all-in-one application for Web purposes. With the onset of app stores our program usage has gotten increasingly more fragmented. SeaMonkey unifies all of your Internet applications in a single app, helping you to maintain a clean and minimalistic way of using your computer online.
Extension compatibility: One good thing about SeaMonkey is that it should make for an easy switch for users of Thunderbird or Firefox, since it supports a large number of the extensions for those two apps. Not every extension works, but enough do that you should be able to use most of your favorite extensions from those other apps with SeaMonkey.
Overall design feel: The main problem with SeaMonkey is that it isn't as up to date in its design as some of the other, more commonly used browsers. Its overall feel, in both the browser window and the navigation buttons and tools, is reminiscent of other browsers from five or more years ago.
Although SeaMonkey looks a little out of fashion, it is still a very useful app. If you want an integrated email and browser, then SeaMonkey should definitely be on your download list.