Do you mean the "remember this login for 30 days" setting?
If you want lastpass to remember this setting then you have configure firefox to store the lastpass extension cookie. The "remember 30 days"setting is stored in this cookie.
Even with the lastpass extension version 4.2.4a this seems to be true.
See my other post for configuring firefox
viewtopic.php?f=12&t=283685