Is this different from what happens when you've restored a bunch of tabs from a previous session, and those tabs don't really load until you visit them for the first time? This happens even after you've visited the tab during the current session?

Firefox allows add-ons to "suspend" tabs, which could lead to those tabs having to reload on the next visit, but I don't think Firefox suspends or unloads tabs on its own. Maybe there is a setting for that??

No, these are just tabs behind other tabs inactive for a while, and I don't see any plugin or extension to blame, (like the The Great Suspender on Chrome). Some web pages have elaborate internal setup, so reload is costly.