"kfmclient exec,gnome-open,kmail,mozilla-thunderbird,thunderbird,evolution";
A Gnome user will be offended if you spool up the entire KDE environment just to launch a URL; a KDE user will be similarly offended by gnome-open. Further, if I'm a Gnome user, I may have KDE on my system, and kfmclient exec may work, but it's likely to not be the browser I *really* want used.
The standard that's developing is that you don't invoke a KDE program unless KDE_FULL_DESKTOP is set and you don't invoke a Gnome session unless GNOME_DESKTOP_SESSION_ID is set.
Cheers,
Jeremy