I put together a modified version of the TabsOpenRelative add-on which combines the features of that add-on with a fix for the removal of separate preferences for external links in Firefox 3.5. See the links below for more details.
It is also the first thing to be posted to soft.world3.net, where I will be putting all my little bits of code once they are fit for release.
TabsOpenRelative (modified) is available here: https://addons.mozilla.org/en-US/firefox/addon/13626 and http://soft.world3.net/tabsopenrelative.html