LastPass offers more safety than the built-in password manager in Firefox and that's the reason why LastPass is being installed by lots of users nowadays.
LastPass stores passwords in its own server and encrypts them before they are uploaded to the server, so they are not accessible on the way.
Of course, we recommend you to set an strange but easy-to-remember password, because that one will give you access to all your passwords.
The other advantage is that it can create high-security passwords you won't have to remember (it will use them automatically). By that way you'll increase security but you won't have to rack your brains to remember them.
Finally, you can import all passwords from Firefox automatically. Don't hesitate, safety is important and LastPass increases it.