I have been looking into the SeaMonkey browser myself so let me ask a few questions. I thought Firefox let you put tabs below the menu bar and that the menu bar can be made into a standard list of File, Edit, etc. instead of that big orange Firefox button. What does SeaMonkey offer that Firefox (I use Pale Moon) does not have? Do you use the other features besides the browser in the SeaMonkey Suite?
I only use SeaMonkey because I'm stubborn against new layouts. I also use their mail client for email, which, again, I use because I'm stubborn. I used the mail client in Netscape 4, then in Mozilla, and I never liked the look of thunderbird so I kept with Mozilla as long as I could, then came across SeaMonkey.
The browser rendering is firefox, and it gets updated with each update of firefox (once or twice a month there's an update that auto-installs). It has all the bells and whistles of a modern browser. You can install firefox extensions into SeaMonkey, but only some of them. Maybe half work, half don't.
There are a few other differences between firefox & seamonkey, such as how you open a new link, if it's a popup window going into a new tab, or you specify the link to open in a new tab, if that tab automatically takes the focus, how to close tabs down, little stuff that I just preferred to stick with what I was used to.
It's all about look & feel. Pick the browser who's interface you are most comfortable with.