You can get Firefox to open all the tabs you last had open when it starts up, this way you can close and open Firefox whenever you want, and you'll always be right back where you started.

This doesn't save the page state, so if you were filling in a form, you'll lose that information, but it does mean that wherever you reopen Firefox, you end up pretty much exactly where you were when you closed it.

I guess you'd still need to regularly close and reopen the browser as a whole (I do that at end of the day anyway when I turn my machine off), but it may be more convenient than losing state.