Something to be aware of Rated 1 out of 5 stars
According to Heisse Security @ h-online.com, anyone who attempts to test LastPass security will have their IP address blocked. The problem here is that if another user is using the same proxy or NAT firewall, they'll also get a message that their IP has been blocked.
Also, as android272 points out, FF already has a very good password manager which makes this add-on superfluous.
Similarly, "KeePass" offers an excellent password manager and is worth checking out. Point FF to "keepass.info" (without quotes) for more info.