I use both SeaMonkey and Firefox -- Firefox as default browser, but clearing cookies and other info between sessions, and SeaMonkey for email and browsing with persistent cookies -- but the browser I use most of the time is Opera anyway.
The browser in SeaMonkey 2.0 is essentially Firefox 3.5.4 with a SeaMonkey look to it (and SeaMonkey type preferences) and the email client is closer to Thunderbird, so the memory footprint is equivalent to either of those, but if you use both the email client and browser at the same time, you'll see substantial savings.
My checks showed that Seamonkey 1.1.18 browser by itself used about 20MB RAM on startup, Firefox about 32MB, and Seamonkey 2.0 about 30MB for either the browser or email client alone, but only about 38MB for both at the same time. I didn't do any serious testing to see how they compared when actually loading web sites, although sites that worked with Firefox but not SeaMonkey 1.1.x do work with 2.0
One thing to be aware of is that 2.0 stores profile data like Firefox and Thunderbird, not like 1.1.x. If, as I do, you store your data in a non-standard location to allow easy syncing between computers, you'll need to take some extra steps. On first run, let SeaMonkey import everything from your 1.1.x profile into the new profile location, then move the contents of the new profile folder where you want it to reside and edit profile.ini in %APPDATA%/Mozilla/SeaMonkey to point there. See
http://support.mozil...+up+your+information for more information on moving profiles.