I use Session Manager [1] for this. It has a "restore tabs on demand" option. I usually end up restarting the browser every week or two, either due to Windows updates, Firefox updates, or getting enough active tabs that the browser crashes. When it reloads, I only have one loaded tab, and start the cycle over again.
I think the lazy loading on session restore has been built in to Firefox for a while now, but I still use Session Manager. It shows me the number of tabs in the session before I choose to restore it, and if I hit a problematic page that is making the browser immediately crash, I can remove that page from the session before restoring, as well.
