Lately, Lastpass has occasionally struggled with the add-on. The currently version (4.9.2) for Firefox is broken. You need to roll back to version 4.9.1.1 and things will work as expected. See this forum post: https://forums.lastpass.com/viewtopic.php?f=12&t=299455&start=30#p999975
For those complaining about the loss of functionality compared to the older versions (ability to easily copy passwords/usernames), you need to install the "Binary Component" which is NOT part of the new plugin but can easily be installed. See: https://lastpass.com/support.php?cmd=showfaq&id=5576
