Hi, is the question coming from the site in its own little panel, or from Firefox?
If you saved a site block in Firefox, it should be saved from session to session unless (A) it's a private window -- in that case, it's not saved to disk to avoid creating a trail of sites visited -- or (B) if Firefox is set to clear history when you close it and you included the "Site Settings"/"Site Preferences" category in the list of what should be cleared.
