I'm slightly confused by the Seamonkey thing but I gather that project is to continue development of the existing Mozilla suite? I'm downloading a copy now to see if it's much different from Mozilla 1.x (
http://www.mozilla.org/products/mozilla1.x/ ).
But anyway, these are the reasons why I use the entire suite instead of just Firefox + Thunderbird, even though the browser and mail are the only components of the suite that I bother to use:
-I started using it before Firefox came out and got used to it, leading to the next one:
-Firefox's interface is similar to Mozilla's but has just enough small differences to drive me batshit crazy (for example, in Mozilla if you right-click at the top of a tab, the first option in the menu is "Close tab" and I use this constantly and almost unconsciously to close tabs I'm done with; in Firefox the first option is "New tab" instead, and last time I tried Firefox I thought I was losing my mind because I had all these new blank tabs and the old tabs were still there). If you're used to Firefox, you might find the opposite to be true for the suite.
and
-I hate the way download manager in Firefox works.
If it weren't for those things I'd just use Firefox and Thunderbird since most of the nifty extensions you can find are made for those and don't necessarily work properly in the Mozilla suite, and development is probably going to be more active for them. Sooner or later I'll probably have to switch and ajust to how Firefox works even though it makes me crazy every time I try.
As an aside, I sorta recall the big deal about Firefox and Thunderbird as standalone components was to get rid of bloat. But the download for the suite (the last version I downloaded, anyway, maybe a month or two ago) is 11.4Mb, and the copy of Seamonkey I'm downloading right now is about 12Mb, whereas the download pages for Firefox and Thundebird say they're 5Mb 6Mb respectively. I wonder what's up with that? That's not much difference in size from the entire suite...