I can't recommend the Tree Style Tabs FF add-on enough for these kinds of "large sessions that diverge into many sub-sessions". If you ctrl-click a link, this add-on creates a new child-tab of the current tab and loads the link there. It kind of builds little spanning trees of the WWW graph, which seems to be a very natural way of browsing the web, but you can also organize these tab trees completely freely with drag & drop.

You can select tabs using your normal item selecting techniques, including control and shift.

I wish all browsers would inherit this, that is a very powerful thing.