as a big firefox fan myself, i think FF is the big deal in N900. Anyway, there are some tips I know and always use on my laptop to improve email experience.
firstly, you need to install greasemonkey (FF extention) and restart the browser.
then, for better gmail, install this script http://userscripts.org/scripts/show/52619 which will disable the sponsored links giving you a better look with no extra stuff on the N900 screen.
for windowslive (hotmail) users, you can add this script http://userscripts.org/scripts/show/8228 to remove top and right side ads and free up your N900 screen from annoying ads.
and of course you can find your favorite scripts that will make your frequent visited sites feels and looks better on the N900.

good luck