History:
- First, there was Netscape Navigator, a browser.
- Then came Netscape Communicator, which was an "all in one" product, including a browser, an email & newsgroup client, an address book and an HTML editor.
- Then came Mozilla Application Suite, which was similar to Netscape Communicator.
- Mozilla being an open-source project, not everyone agreed on the direction development should take. One group focused on a browser; that became Mozilla Firefox. Another group focused on email/newsgroups; that became Mozilla Thunderbird. And yet a third group liked the idea of an all in one suite; that project became SeaMonkey.
Basically SeaMonkey is a combination of Firefox and Thunderbird.
When you wrote "what about Firefox? Additionally, I use SeaMonkey," I thought that perhaps you were using both Firefox and Seamonkey. If you're satisfied with SeaMonkey, stay with it, although it's my understanding that SeaMonkey isn't as robustly supported as Firefox and Thunderbird. See, e.g., https://support.mozilla.org/en-US/questions/1200012