Since LastPass has just ended maintenance of their old-fashioned Firefox extension (version 3.x) that worked in SeaMonkey (and Pale Moon), I've decided to make a workaround.
The extension Vault Shortcut adds a button to the toolbar that opens the LastPass Vault in another installed browser - the default is Chrome, and Firefox, Vivaldi, and IE are also preset options. From there you can copy the usernames and passwords to the clipboard and paste them into SeaMonkey.
P.S. Is this the best place to post a SeaMonkey extension? I don't know if SM users would check the Extension/Theme Releases board, and AMO's review queue is so long that one of my other extensions has been in there for two months and still hasn't been approved or denied.