It has tabs. It also has this cool "preview/thumbnail" of the tab when you tooltip the tab label.
It might have varied by OS, what OS are you using. I tested Seamonkey years ago, I can't remember :)
Well yeah, that's why we have Firefox!
The idea of having a multi-module browser/mail/chat/addressbok/news/LDAP application was probably not a good idea. (Does anyone use "works" packages these days?). Netscape had also done a lot of damage to the implementation due to its various commercial requirements. "Mozilla" was the open version of the code that Netscape->AOL funded because it was going to re-implement the functionality in Communicator 4.x. So, it had to have most of everything that Netscape wanted, minus the branding.
But overtime, the community has done a lot of great work to improve Seamonkey. Some of that is work in Gecko, which benefits Firefox as well, but a lot of it is in the Seamonkey side.
Some people thought Mozilla Application Suite would die when mozilla.org switched to developing Firefox-only, but I always believed that a smaller power-user audience would need a swiss-army knife version of the Gecko browser. I just thought Netscape had picked the wrong kind of tools.
A couple examples:
explicit support for profiles. (Firefox added this back later...), but if you test or develop, you can flip back and forth from everything in your profile (cookies, cache, etc).
menu access to Cookie Manager. When I test cookie related problems in Firefox, I keep grumbling about having to go into Prefs just to blow away one thing.
Some of this might be in newer extensions, but Firefox+Thunderbird can never be as strongly integrated as the modules in Seamonkey (after all that was the original appeal of a single multi-module application).
...So give it a try, depending on your work tasks, you might prefer it.