The reason it reloads sites is because many fail to respond to a time check that they haven't changed their content, known as the "304 Not Modified" HTTP header, so Firefox retrieves the page expecting new content.
Firefox is unable to know how slow your connection is, or that you don't care about new content, so can't behave in the way you want automatically, but you can trigger it to load tabs from the cache by doing
File -> Work Offline
Before closing Firefox, and when you restart it, it will still think it's offline and only load tabs from the cache. I don't think toggling it back from Offline afterwards will trigger all the tabs to reload at once. ccie