OK, sorry for the delay, Dave.
Once you have set up the Mail account(s) in SeaMonkey Mail, if you locate your SeaMonkey profile, usually C:\Documents and Settings\[username]\Application Data\Mozilla\Profiles, you should be able to simply copy the Mail folder (or contents of) from C:\Documents and Settings\[username]\Application Data\Thunderbird\Profiles\xxxxxxxx.default\Mail, into the equivalent SeaMonkey location.
Same procedure with Firefox Bookmarks - locate C:\Documents and Settings\[username]\Application Data\Mozilla\Firefox\Profiles\xxxxxxxx.default, and copy the Bookmarks file to the SeaMonkey profile. Job done.
Unfortunately, SM won't clear your cache automatically on exit (I'll look for an add-on for you), but you'll find it does have a better Cookie Manager than Firefox, and is more easily accessible (Tools > Cookie Manager) and controllable.