I'm currently downloading FF13 only because of that feature. I always have 100+ tabs open, and oftentimes, when a browser is "full" (i.e., too many tabs) I switch to another browser until I have time to clean up the mess. There were situations where Chrome, Safari, and Opera were "full".
Really looking forward to this new feature.
Means basically if you open a tab via click or contextual menu and u don't have "only load tabs when selected" it WILL load the tab. But when you restart the browser (or start) with your 100tabs, it will only load the first one. Before that itd both be slow if you had a bad connection and be slow (obviously no browser can instant-render 100 pages)